#' @title Compute Group-Time Average Treatment Effects
#'
#' @description `compute.att_gt` does the main work for computing
#'  multiperiod group-time average treatment effects
#'
#' @param dp A DIDparams object
#'
#' @return a list with length equal to the number of groups times the
#'  number of time periods; each element of the list contains an
#'  object that contains group-time average treatment effect as well
#'  as which group it is for and which time period it is for. It also exports
#'  the influence function which is used externally to compute
#'  standard errors.
#'
#' @keywords internal
#'
#' @export
compute.att_gt <- function(dp) {
  #-----------------------------------------------------------------------------
  # unpack DIDparams
  #-----------------------------------------------------------------------------
  # Internal pretest hook (set only by conditional_did_pretest): when a
  # y-invariant setup bundle is supplied (dp$setup_precomp, built once per
  # pretest call via dp$return_setup below), the bundle replaces the whole
  # per-call setup, including this data.table copy, so skip it.
  setup_precomp <- dp$setup_precomp
  if (is.null(setup_precomp)) data <- data.table::as.data.table(dp$data)
  yname <- dp$yname
  tname <- dp$tname
  idname <- dp$idname
  xformla <- dp$xformla
  weightsname <- dp$weightsname
  est_method <- dp$est_method
  extra_args <- if (is.null(dp$extra_args)) list() else dp$extra_args
  base_period <- dp$base_period
  panel <- dp$panel
  true_repeated_cross_sections <- dp$true_repeated_cross_sections
  print_details <- dp$print_details
  control_group <- dp$control_group
  anticipation <- dp$anticipation
  gname <- dp$gname
  n <- dp$n
  nT <- dp$nT
  nG <- dp$nG
  tlist <- dp$tlist
  glist <- dp$glist

  #-----------------------------------------------------------------------------
  # main computations
  #-----------------------------------------------------------------------------

  # will populate with all att(g,t)
  attgt.list <- list()

  # place holder in lists
  counter <- 1

  # number of time periods
  tlist.length <- if (base_period != "universal") length(tlist) - 1L else length(tlist)
  tfac <- if (base_period != "universal") 1L else 0L

  # influence function dimensions
  inffunc_nrow <- n

  # list to collect sparse matrix updates (built into sparse matrix after loops)
  inffunc_updates <- list()
  # counter for keeping track of updates
  update_counter <- 1

  # never treated option
  nevertreated <- (control_group[1] == "nevertreated")

  fix_weights <- dp$fix_weights

  # intercept-only design (xformla = ~1, the default): the per-cell overlap and
  # rcond guards have closed forms (see overlap_check_fail / rcond_check_fail),
  # so the preliminary logit fit and Gram matrix can be skipped.
  intercept_only <- (xformla == ~1)

  # Cache for the per-cell overlap/rcond guard booleans. For panel data with
  # control_group = "nevertreated" and non-varying weights, the guards'
  # (covariates, G) inputs are bit-identical across all cells of a group that
  # share a covariate (and weight) period, so each boolean is computed once per
  # (g, covariate-period, weight-period) key. Not used for notyettreated (the
  # control cohort varies with t) or fix_weights = "varying" (RC estimator
  # path). Only the booleans are stored; failure warnings are still emitted per
  # cell. options(did.disable_check_cache = TRUE) forces the per-cell checks
  # (escape hatch / used to verify bit-identical equivalence). Mirrors the fast
  # path (compute.att_gt2).
  use_check_cache <- panel && nevertreated &&
    (is.null(fix_weights) || fix_weights != "varying") &&
    !isTRUE(getOption("did.disable_check_cache"))
  if (use_check_cache) check_cache <- new.env(parent = emptyenv())

  # Whether to compute influence functions (default TRUE; FALSE = point estimates
  # only). When FALSE we skip requesting influence functions from the 2x2 estimators,
  # skip the per-cell influence-function scaling, and skip storing inffunc_updates --
  # so a point-estimates-only run does not pay the influence-function compute/memory
  # cost. The att estimates are identical either way.
  do_inf <- is.null(dp$compute_inffunc) || isTRUE(dp$compute_inffunc)

  # Internal pretest-only flag (set by conditional_did_pretest): skip
  # post-treatment (g,t) cells entirely -- the pretest immediately discards
  # them (its keepers filter retains only group > t cells), so computing them
  # is pure waste. attgt.list and the influence-function columns stay aligned
  # with the reduced cell set because the counters only increment when a cell
  # is appended.
  pretreatment_only <- isTRUE(dp$pretreatment_cells_only)

  if (!is.null(setup_precomp)) {
    # ---- internal pretest path: reuse the y-invariant setup bundle -----------
    # conditional_did_pretest() calls compute.att_gt once per unit with only
    # the outcome column changed, so the whole per-call setup below (column
    # extraction, design matrix, per-period slicing, alignment check) is built
    # once via dp$return_setup and reused; each per-unit call passes only
    # dp$y_override (that unit's modified outcome, aligned 1:1 with dp$data
    # rows). The bundle is consumed only when the balanced, id-aligned panel
    # precompute applies (conditional_did_pretest keeps its legacy data-copy
    # path otherwise), and that path never reads `data` / `g_col` / `t_col`
    # inside the loops -- they are deliberately left undefined here so any
    # accidental read fails loudly instead of silently using a stale outcome.
    use_precompute_rc <- FALSE
    use_precompute_panel <- isTRUE(setup_precomp$use_precompute_panel)
    weights_by_period <- setup_precomp$weights_by_period
    period_mm <- setup_precomp$period_mm
    period_id <- setup_precomp$period_id
    period_w  <- setup_precomp$period_w
    g_unit_panel <- setup_precomp$g_unit_panel
    if (nevertreated) C_full <- setup_precomp$C_full
    # only the outcome differs across pretest iterations: rebuild the
    # per-period outcome slices from the override (bit-identical to slicing a
    # full data copy whose outcome column was overwritten, which is what the
    # legacy pretest loop did)
    y_override <- dp$y_override
    period_y <- lapply(setup_precomp$rows_by_period, function(rr) y_override[rr])
  } else {
    # Pre-extract columns to avoid repeated get() inside data.table (which is slow)
    g_col <- data[[gname]]
    t_col <- data[[tname]]

    # Build weight lookup by period for fix_weights options (balanced panel only)
    if (!is.null(fix_weights) && panel) {
      weights_by_period <- list()
      for (tp in seq_along(tlist)) {
        weights_by_period[[tp]] <- data[t_col == tlist[tp], .w]
      }
    }

    # Positional precompute for the repeated-cross-section / unbalanced-panel path:
    # each (g,t) cell is assembled from plain vectors and per-period row indices
    # precomputed once per call, instead of two full data.table subsets plus a
    # full-data %in% per cell (the dominant per-cell cost on this path). Bit-identical
    # to the legacy construction (see the per-cell comments below);
    # options(did.disable_precompute = TRUE) forces the legacy per-cell assembly
    # (escape hatch / used to verify bit-identical cell assembly; both sub-paths
    # still slice the hoisted global design matrix), mirroring the panel precompute.
    use_precompute_rc <- !panel && !isTRUE(getOption("did.disable_precompute"))

    # The precompute paths never read data$.C / data$.G (cohort indicators are
    # rebuilt from plain vectors), so skip the full-length column writes there.
    if (nevertreated && !use_precompute_rc) {
      set(data, j = ".C", value = as.integer(g_col == 0))
    }

    # -- design matrix: built ONCE over the full data, then row-subset per (g,t) cell --
    # This (a) avoids rebuilding model.matrix() nG*nT times, and (b) gives every cell the
    # GLOBAL factor levels / GLOBAL transform basis -- exactly what the repeated-cross-
    # section path, the fast path, and manually-constructed dummy columns use. As a
    # result a factor covariate behaves identically to adding its dummies by hand: a
    # level absent from a particular 2x2 comparison becomes an all-zero column (handled
    # by the estimator's overlap/rank check, giving the same NA + warning as the manual
    # dummies) rather than a dropped contrast that changes the design across cells.
    #
    # For purely row-wise numeric formulae (e.g. ~X, ~X1+X2, ~X1*X2, ~I(X^2)) the slice
    # is bit-identical to a per-cell model.matrix(xformla, disdat); for factors it equals
    # the manual-dummy design; for data-dependent bases (poly/scale/ns/...) it uses the
    # global basis, which yields the same ATT/SE as a per-cell fit (the estimator is
    # invariant to a full-rank reparameterization of the covariates).
    #
    # `data` is already complete-cases and finite (see pre_process_did), so na.pass adds
    # no NA rows and full_mm aligns 1:1 (by row position) with `data`.
    #
    # Drop globally-empty factor levels first: model.matrix() emits a dummy column for
    # every declared level, so a level absent from the ENTIRE estimation sample (common
    # after users subset their data) would produce an all-zero column in every cell and
    # NA-fail the whole estimation. Levels absent only from a particular 2x2 cell are
    # unaffected (still all-zero in that cell only, as documented above). Mirrors the
    # identical fix in get_did_tensors() (fast path).
    for (v in all.vars(xformla)) {
      if (is.factor(data[[v]])) set(data, j = v, value = droplevels(data[[v]]))
    }
    full_mm <- model.matrix(xformla, data = data, na.action = na.pass)
    if (panel) {
      # per-period row slices keyed by idname, for the earlier-period covariates that
      # get_wide_data() retains. (idname is used rather than .rowid because .rowid is
      # only created on the RC/unbalanced path; in a balanced panel idname is unique
      # within each period.)
      period_mm <- vector("list", length(tlist))
      period_id <- vector("list", length(tlist))
      period_y  <- vector("list", length(tlist))
      period_w  <- vector("list", length(tlist))
      # per-period row indices, kept for the pretest setup bundle (dp$return_setup)
      rows_by_period <- vector("list", length(tlist))
      y_col <- data[[yname]]
      w_col <- data[[".w"]]
      for (tp in seq_along(tlist)) {
        rows_tp <- which(t_col == tlist[tp])
        rows_by_period[[tp]] <- rows_tp
        period_mm[[tp]] <- full_mm[rows_tp, , drop = FALSE]
        period_id[[tp]] <- data[[idname]][rows_tp]
        period_y[[tp]]  <- y_col[rows_tp]
        period_w[[tp]]  <- w_col[rows_tp]
      }
      # Precompute fast path: when the panel is balanced with every period sharing the
      # same units in the same (id-sorted) row order, each 2x2 cell can be assembled by
      # position from these per-period blocks -- skipping the per-cell data[time_mask]
      # subset + get_wide_data() reshape (the dominant cost on the slow path).
      # fix_weights = "base_period"/"first_period" only changes WHICH period's weight
      # vector each cell uses (selected from weights_by_period below, mirroring the
      # fallback), so the precompute applies there too. Falls through to the original
      # get_wide_data() code, bit-identically, for fix_weights = "varying" (which needs
      # the long-format RC data) / unaligned panels.
      panel_units_aligned <- all(vapply(period_id, function(x)
        length(x) == length(period_id[[1L]]) && all(x == period_id[[1L]]), logical(1)))
      g_unit_panel <- g_col[which(t_col == tlist[1L])]
      # options(did.disable_precompute = TRUE) disables only this per-cell positional
      # assembly, falling back to the per-cell get_wide_data() reshape below (escape
      # hatch / used to verify bit-identical cell assembly). It does NOT bypass the
      # design-matrix hoist: BOTH sub-paths slice the global full_mm/period_mm built
      # above, so a hoist regression would be invisible to this option -- the hoist
      # itself is validated by the master-comparison tests in
      # test-modelmatrix-hoist.R, not by this escape hatch.
      use_precompute_panel <- (is.null(fix_weights) ||
        fix_weights %in% c("base_period", "first_period")) && panel_units_aligned &&
        !isTRUE(getOption("did.disable_precompute"))
      # Never-treated control cohort is invariant across every (g,t) cell, so build
      # it once here; notyettreated rebuilds C_full per cell inside the loops.
      if (use_precompute_panel && nevertreated) {
        C_full <- as.numeric(g_unit_panel == 0)
      }
    } else {
      # repeated cross-section / unbalanced: row-subset the full design by POSITION
      # (.rowid is not unique across periods here).
      global_mm <- full_mm
      use_precompute_panel <- FALSE
      if (use_precompute_rc) {
        # One-time positional precompute: per-period row indices into `data` plus
        # plain copies of the columns each cell consumes. which() preserves data
        # order within each period, so per-period vectors are bit-identical to the
        # corresponding data.table period subsets.
        period_rows_rc <- vector("list", length(tlist))
        for (tp in seq_along(tlist)) {
          period_rows_rc[[tp]] <- which(t_col == tlist[tp])
        }
        rowid_col <- data$.rowid
        y_col_rc <- data[[yname]]
        w_col_rc <- data[[".w"]]
        # loop-invariant id universe for the influence-function index lookup
        unique_rowid <- unique(rowid_col)
        # fix_weights base/first: per-period .rowid/.w lookup vectors replace the
        # per-cell full-table data[t_col == target_period, ] subset; match()
        # semantics (first match wins, unmatched -> NA) are unchanged.
        if (!is.null(fix_weights) && fix_weights %in% c("base_period", "first_period")) {
          target_rowid_by_period <- lapply(period_rows_rc, function(rr) rowid_col[rr])
          target_w_by_period <- lapply(period_rows_rc, function(rr) w_col_rc[rr])
        }
      }
    }
  }

  # Internal (set only by conditional_did_pretest): return the y-invariant
  # setup bundle built above -- so the pretest can hoist it out of its
  # per-unit loop -- instead of estimating. Balanced panel only; the pretest
  # keeps its legacy data-copy loop for repeated cross sections, and only
  # consumes the bundle when use_precompute_panel is TRUE.
  if (isTRUE(dp$return_setup)) {
    if (!panel) return(NULL)
    return(list(
      use_precompute_panel = use_precompute_panel,
      weights_by_period = if (!is.null(fix_weights)) weights_by_period else NULL,
      period_mm = period_mm,
      period_id = period_id,
      period_w = period_w,
      rows_by_period = rows_by_period,
      g_unit_panel = g_unit_panel,
      C_full = if (use_precompute_panel && nevertreated) C_full else NULL
    ))
  }

  # loop over groups
  for (g in 1:nG) {
    # Set up .G once (use pre-extracted g_col to avoid get() inside data.table).
    # The precompute paths derive the treated cohort from g_unit_panel / g_col
    # directly and never read data$.G, so skip the full-length column write there.
    current_g <- glist[g]
    if (!use_precompute_panel && !use_precompute_rc) {
      set(data, j = ".G", value = as.numeric(g_col == current_g))
    }

    # pre-compute the universal/post-treatment base period for this group (used multiple times)
    idx_g <- which((tlist + anticipation) < glist[g])
    pret_g <- if (length(idx_g) == 0L) NA_integer_ else idx_g[length(idx_g)]

    # Precompute path: the treated-cohort indicator depends only on g, so build it
    # once per group. Under nevertreated the control cohort is also t-invariant
    # (C_full was hoisted above the loops), so the kept-unit index and the G/C
    # vectors are hoisted here too; notyettreated recomputes C_full/disidx/kept
    # per cell below because its control cohort varies with t.
    if (use_precompute_panel) {
      G_full <- as.numeric(g_unit_panel == current_g)
      if (nevertreated) {
        disidx <- G_full == 1 | C_full == 1
        kept <- which(disidx)
        n1 <- length(kept)
        G <- G_full[kept]
        C <- C_full[kept]
      }
    }

    # loop over time periods
    for (t in 1:tlist.length) {
      #-----------------------------------------------------------------------------
      # Set pret

      # varying base period
      pret <- t

      # universal base period
      if (base_period == "universal") {
        # use same base period as for post-treatment periods
        pret <- pret_g
      }

      # check if in post-treatment period
      if ((glist[g] <= tlist[(t + tfac)])) {
        # update pre-period if in post-treatment period to
        # be  period (g-delta-1)
        pret <- pret_g
      }

      # check if there are no pre-treatment periods
      if (is.na(pret)) {
        warning(paste0("There are no pre-treatment periods for the group first treated at ", glist[g], "\nUnits from this group are dropped"))
        break
      }

      # pretest-only: skip post-treatment cells entirely (see
      # pretreatment_only above; conditional_did_pretest discards these
      # cells unread, so none of the per-cell work below is needed)
      if (pretreatment_only && glist[g] <= tlist[t + tfac]) next

      # use "not yet treated as control"
      # that is, never treated + units that are eventually treated,
      # but not treated by the current period (+ anticipation)
      if (!nevertreated) {
        # Use pre-extracted g_col to avoid get() inside data.table.
        # The precompute paths recompute the control cohort from
        # g_unit_panel / g_col and never read data$.C, so skip the full-length
        # column write there (the time_threshold scalar is still needed below).
        time_threshold <- tlist[max(t, pret) + tfac] + anticipation
        if (!use_precompute_panel && !use_precompute_rc) {
          set(data, j = ".C", value = as.integer((g_col == 0) |
            ((g_col > time_threshold) &
              (g_col != current_g))))
        }
      }


      #-----------------------------------------------------------------------------
      # if we are in period (g-1), normalize results to be equal to 0
      # and break without computing anything
      if (base_period == "universal") {
        if (tlist[pret] == tlist[(t + tfac)]) {
          attgt.list[[counter]] <- list(att = 0, group = glist[g], year = tlist[(t + tfac)], post = 0)
          # inffunc[,counter] <- rep(0,n)
          # counter <- counter+1
          if (do_inf) inffunc_updates[[update_counter]] <- list(
            indices = integer(0), # No non-zero entries
            values = numeric(0)
          )

          # Update the counters
          update_counter <- update_counter + 1
          counter <- counter + 1
          next
        }
      }

      # print the details of which iteration we are on
      if (print_details) {
        cat(paste("current period:", tlist[(t + tfac)]), "\n")
        cat(paste("current group:", glist[g]), "\n")
        cat(paste("set pretreatment period to be", tlist[pret]), "\n")
      }

      #-----------------------------------------------------------------------------
      # results for the case with panel data
      #-----------------------------------------------------------------------------

      # post treatment dummy variable
      post.treat <- 1 * (glist[g] <= tlist[t + tfac])

      # total number of units (not just included in G or C)
      # disdat <- data[data[,tname] == tlist[t+tfac] | data[,tname] == tlist[pret],]
      # The precompute paths (panel per-period blocks; RC/unbalanced positional
      # assembly) build each cell without this long 2-period subset (the dominant
      # per-cell cost); the legacy escape-hatch paths still build disdat as before.
      # time_mask is only read here and in the fix_weights == "varying" branch,
      # which always runs with use_precompute_panel == FALSE (the panel precompute
      # excludes fix_weights == "varying") and panel == TRUE (so use_precompute_rc
      # is FALSE), so it is computed on every path that consumes it.
      if (!(panel && use_precompute_panel) && !use_precompute_rc) {
        target_times <- c(tlist[t + tfac], tlist[pret])
        time_mask <- t_col %in% target_times
        disdat <- data[time_mask]
      }


      if (panel) {
        if (use_precompute_panel) {
          # ---- precompute fast path (balanced, id-aligned panel; non-varying weights) --
          # Build the 2x2 cell directly from the per-period blocks. g is time-invariant
          # so .G / .C are unit-level; Ypost is always period (t+tfac) and Ypre always
          # period pret because get_wide_data's .y1/.y0 (later/earlier) swap exactly
          # cancels the universal-base ordering used below. The earlier period (whose
          # weights + covariates get_wide_data retains) is min(t + tfac, pret).
          # G_full (and, under nevertreated, C_full/disidx/kept/n1/G/C) are hoisted
          # to the group/setup level above; only t-varying pieces are built here.
          if (!nevertreated) {
            # time_threshold was computed above (kept for the precompute path)
            C_full <- as.numeric((g_unit_panel == 0) |
              ((g_unit_panel > time_threshold) & (g_unit_panel != current_g)))
            disidx <- G_full == 1 | C_full == 1
            kept <- which(disidx)
            n1 <- length(kept)
            G <- G_full[kept]
            C <- C_full[kept]
          }
          n <- length(g_unit_panel)
          Ypost <- period_y[[t + tfac]][kept]
          Ypre  <- period_y[[pret]][kept]
          earlier_idx <- min(t + tfac, pret)
          # Select weights based on fix_weights, mirroring the fallback's
          # weights_by_period[[...]][disidx] selection bit-identically:
          # weights_by_period[[tp]] and period_w[[tp]] share the same within-period
          # data order, and kept == which(disidx) under panel_units_aligned.
          if (is.null(fix_weights)) {
            # Default: the earlier period's weights (what get_wide_data retains)
            w <- period_w[[earlier_idx]][kept]
          } else if (fix_weights == "base_period") {
            w <- weights_by_period[[pret_g]][kept]
          } else {
            # fix_weights == "first_period" ("varying" never reaches the precompute)
            w <- weights_by_period[[1L]][kept]
          }
          covariates <- period_mm[[earlier_idx]][kept, , drop = FALSE]
          dimnames(covariates) <- list(NULL, colnames(covariates))
        } else {
          # ---- original path: per-cell get_wide_data() reshape ----
          # transform  disdat it into "cross-sectional" data where one of the columns
          # contains the change in the outcome over time.
          disdat <- get_wide_data(disdat, yname, idname, tname)

          # still total number of units (not just included in G or C)
          n <- nrow(disdat)

          # pick up the indices for units that will be used to compute ATT(g,t)
          disidx <- disdat$.G == 1 | disdat$.C == 1

          # pick up the data that will be used to compute ATT(g,t)
          disdat <- disdat[disidx]

          n1 <- nrow(disdat) # num obs. for computing ATT(g,t)

          # drop missing factors
          disdat <- droplevels(disdat)

          # give short names for data in this iteration
          G <- disdat$.G
          C <- disdat$.C

          # handle pre-treatment universal base period differently
          # we need to do this because panel2cs2 just puts later period
          # in .y1, but if we are in a pre-treatment period with a universal
          # base period, then the "base period" is actually the later period
          Ypre <- if (tlist[(t + tfac)] > tlist[pret]) disdat$.y0 else disdat$.y1
          Ypost <- if (tlist[(t + tfac)] > tlist[pret]) disdat$.y1 else disdat$.y0

          # Select weights based on fix_weights
          if (is.null(fix_weights)) {
            # Default: .w from get_wide_data (earlier period)
            w <- disdat$.w
          } else if (fix_weights == "base_period") {
            w <- weights_by_period[[pret_g]][disidx]
          } else if (fix_weights == "first_period") {
            w <- weights_by_period[[1L]][disidx]
          } else if (fix_weights == "varying") {
            w <- disdat$.w  # will be overridden below when switching to RC estimator
          } else {
            w <- disdat$.w
          }

          # matrix of covariates: index the precomputed earlier-period design by this
          # cell's units (idname). get_wide_data retained the earlier period, whose
          # index is min(t + tfac, pret).
          earlier_idx <- min(t + tfac, pret)
          covariates <- period_mm[[earlier_idx]][match(disdat[[idname]], period_id[[earlier_idx]]), , drop = FALSE]
          dimnames(covariates) <- list(NULL, colnames(covariates))
        }

        #-----------------------------------------------------------------------------
        # code for actually computing att(g,t)
        #-----------------------------------------------------------------------------

        attgt <- tryCatch({
          if (!is.null(fix_weights) && fix_weights == "varying") {
            # fix_weights = "varying": use RC estimators with per-period weights
            # Go back to long-format data for this (g,t) cell
            disdat_long <- data[time_mask]
            disdat_long_idx <- disdat_long$.G == 1 | disdat_long$.C == 1
            disdat_long <- droplevels(disdat_long[disdat_long_idx])
            Y_rc <- disdat_long[[yname]]
            G_rc <- disdat_long$.G
            post_rc <- as.numeric(disdat_long[[tname]] == tlist[t + tfac])
            w_rc <- disdat_long$.w
            # Use earlier-period covariates for all observations — fix_weights
            # only changes weights, not the covariate conditioning set.
            # Use min(pret, t+tfac) to match the panel estimator's convention:
            # with base_period="universal", pret can be later than t for placebo cells.
            earlier_idx_v <- min(pret, t + tfac)
            earlier_period <- tlist[earlier_idx_v]
            early_mask <- disdat_long[[tname]] == earlier_period
            disdat_early <- disdat_long[early_mask]
            cov_early <- period_mm[[earlier_idx_v]][match(disdat_early[[idname]], period_id[[earlier_idx_v]]), , drop = FALSE]
            dimnames(cov_early) <- list(NULL, colnames(cov_early))
            # Map each row in disdat_long to its unit's earlier-period covariates
            early_ids <- disdat_early[[idname]]
            all_ids <- disdat_long[[idname]]
            id_map <- match(all_ids, early_ids)
            covariates_rc <- cov_early[id_map, , drop = FALSE]

            # Run overlap/rank checks on RC data (not wide panel data)
            if (!is.function(est_method)) {
              if (est_method %in% c("dr", "ipw")) {
                if (overlap_check_fail(covariates_rc, G_rc, intercept_only)) {
                  warning(paste0("overlap condition violated for group ", glist[g], " in time period ", tlist[t + tfac]))
                  stop("overlap")
                }
              }
              if (est_method %in% c("dr", "reg")) {
                if (rcond_check_fail(covariates_rc, G_rc, intercept_only)) {
                  warning(paste0("Covariate matrix for control units is singular or numerically ill-conditioned for group ", glist[g], " in time period ", tlist[t + tfac], "; consider centering/rescaling covariates or removing collinear terms"))
                  stop("singular")
                }
              }
            }

            if (inherits(est_method, "function")) {
              res <- do.call(est_method, c(list(
                y = Y_rc, post = post_rc,
                D = G_rc, covariates = covariates_rc,
                i.weights = w_rc, inffunc = do_inf
              ), extra_args))
            } else if (est_method == "ipw") {
              res <- DRDID::std_ipw_did_rc(Y_rc, post_rc, G_rc,
                covariates = covariates_rc,
                i.weights = w_rc, boot = FALSE, inffunc = do_inf)
            } else if (est_method == "reg") {
              res <- DRDID::reg_did_rc(Y_rc, post_rc, G_rc,
                covariates = covariates_rc,
                i.weights = w_rc, boot = FALSE, inffunc = do_inf)
            } else {
              res <- DRDID::drdid_rc(Y_rc, post_rc, G_rc,
                covariates = covariates_rc,
                i.weights = w_rc, boot = FALSE, inffunc = do_inf)
            }
          } else {
            # Panel path: run overlap/rank checks on panel data. Guard booleans
            # are reused from the cache when this cell's (covariates, G) inputs
            # are bit-identical to an earlier cell's (nevertreated + non-varying
            # weights; see the cache setup above); warnings are still emitted
            # per cell, so cached failures keep counts and texts unchanged.
            if (!is.function(est_method)) {
              guard <- NULL
              if (use_check_cache) {
                w_idx_key <- if (is.null(fix_weights)) earlier_idx
                             else if (fix_weights == "base_period") pret_g
                             else 1L
                check_key <- paste(g, earlier_idx, w_idx_key, sep = "_")
                guard <- check_cache[[check_key]]
              }
              if (is.null(guard)) {
                # overlap check for pscore based methods; regression-feasibility
                # check for outcome-regression methods, run only if overlap
                # passed (matching the stop("overlap") short-circuit below)
                overlap_fail <- (est_method %in% c("dr", "ipw")) &&
                  overlap_check_fail(covariates, G, intercept_only)
                rcond_fail <- !overlap_fail && (est_method %in% c("dr", "reg")) &&
                  rcond_check_fail(covariates, G, intercept_only)
                guard <- list(overlap_fail = overlap_fail, rcond_fail = rcond_fail)
                if (use_check_cache) check_cache[[check_key]] <- guard
              }
              if (guard$overlap_fail) {
                warning(paste0("overlap condition violated for group ", glist[g], " in time period ", tlist[t + tfac]))
                stop("overlap")
              }
              if (guard$rcond_fail) {
                warning(paste0("Covariate matrix for control units is singular or numerically ill-conditioned for group ", glist[g], " in time period ", tlist[t + tfac], "; consider centering/rescaling covariates or removing collinear terms"))
                stop("singular")
              }
            }

            if (inherits(est_method, "function")) {
              # user-specified function
              res <- do.call(est_method, c(list(
                y1 = Ypost, y0 = Ypre,
                D = G,
                covariates = covariates,
                i.weights = w,
                inffunc = do_inf
              ), extra_args))
            } else if (est_method == "ipw") {
              # inverse-probability weights
              res <- DRDID::std_ipw_did_panel(Ypost, Ypre, G,
                covariates = covariates,
                i.weights = w,
                boot = FALSE, inffunc = do_inf
              )
            } else if (est_method == "reg") {
              # regression
              res <- DRDID::reg_did_panel(Ypost, Ypre, G,
                covariates = covariates,
                i.weights = w,
                boot = FALSE, inffunc = do_inf
              )
            } else {
              # doubly robust, this is default
              res <- DRDID::drdid_panel(Ypost, Ypre, G,
                covariates = covariates,
                i.weights = w,
                boot = FALSE, inffunc = do_inf
              )
            }
          }

          # adjust influence function to account for only using
          # subgroup to estimate att(g,t) (skipped for point estimates only)
          if (do_inf) {
            if (!is.null(fix_weights) && fix_weights == "varying") {
              # RC influence function has one entry per obs in disdat_long
              # (2 per unit: pre + post). Aggregate to unit level by ID,
              # independent of row ordering.
              res$att.inf.func <- as.numeric(rowsum(res$att.inf.func,
                                                    disdat_long[[idname]],
                                                    reorder = FALSE))
              res$att.inf.func <- (n / n1) * res$att.inf.func
            } else {
              res$att.inf.func <- (n / n1) * res$att.inf.func
            }
          }

          # If ATT is NaN, replace it with NA, and mark influence function as missing.
          if (is.nan(res$ATT)) {
            res$ATT <- NA
            if (do_inf) res$att.inf.func <- rep(NA_real_, length(res$att.inf.func))
          }
          res
        }, error = function(e) {
          # The overlap/rank guards above already emitted a full diagnostic warning
          # before signalling the internal "overlap"/"singular" sentinels, so skip
          # the wrapper warning for those: each failed cell warns exactly once,
          # matching the fast path. Genuine estimator errors are still surfaced.
          if (!(conditionMessage(e) %in% c("overlap", "singular"))) {
            warning("Error computing internal 2x2 DiD for (g, t) = (", glist[g], ", ", tlist[t + tfac], "): ", e$message, ". The ATT for this cell will be set to NA.")
          }
          NULL
        })

        if (is.null(attgt)) {
          attgt.list[[counter]] <- list(att = NA, group = glist[g], year = tlist[(t + tfac)], post = post.treat)
          if (do_inf) inffunc_updates[[update_counter]] <- list(
            indices = seq_len(n),
            values = rep(NA_real_, n)
          )
          update_counter <- update_counter + 1
          counter <- counter + 1
          next
        }
      } else { # repeated cross sections / unbalanced panel

        if (use_precompute_rc) {
          # ---- positional precompute path (default) ----
          # .G / .C are row-wise functions of g_col, and gname is time-invariant
          # within .rowid (pre_process_did stops otherwise; for true repeated
          # cross sections .rowid is the row number), so a unit satisfies
          # .G == 1 | .C == 1 either in BOTH of this cell's periods or in
          # NEITHER. The legacy rightids/%in% construction below therefore
          # reduces to a row-wise G|C filter over the two periods, assembled
          # here positionally from the vectors precomputed before the loops --
          # skipping the two full data.table subsets, the full-data %in%, and
          # droplevels() per cell (covariates come positionally from global_mm,
          # so factor levels in `data` are never read). sort() restores original
          # data order, so the rows match which(disidx) on the legacy path
          # bit-identically. (The two periods are always distinct here: the
          # universal-base cell with tlist[pret] == tlist[t + tfac] is skipped
          # before this point.)
          rows_pair <- sort(c(period_rows_rc[[t + tfac]], period_rows_rc[[pret]]))
          g_cell <- g_col[rows_pair]
          Gv <- as.numeric(g_cell == current_g)
          # same formulas and types as the .G/.C column writes on the legacy path
          Cv <- if (nevertreated) {
            as.integer(g_cell == 0)
          } else {
            as.integer((g_cell == 0) |
              ((g_cell > time_threshold) & (g_cell != current_g)))
          }
          sel <- Gv == 1 | Cv == 1
          # positional index into `data` for slicing the global design matrix,
          # kept in sync with any later fix_weights row drops below.
          disdat_rows <- rows_pair[sel]
          G <- Gv[sel]
          C <- Cv[sel]
          Y <- y_col_rc[disdat_rows]
          post <- 1 * (t_col[disdat_rows] == tlist[t + tfac])
          cell_rowid <- rowid_col[disdat_rows]
        } else {
          # ---- legacy path (options(did.disable_precompute = TRUE)) ----
          # pick up the indices for units that will be used to compute ATT(g,t)
          # these conditions are (1) you are observed in the right period and
          # (2) you are in the right group (it is possible to be observed in
          # the right period but still not be part of the treated or control
          # group in that period here
          rightids <- disdat$.rowid[disdat$.G == 1 | disdat$.C == 1]

          # this is the fix for unbalanced panels; 2nd criteria shouldn't do anything
          # with true repeated cross sections, but should pick up the right time periods
          # only with unbalanced panel
          disidx <- (data$.rowid %in% rightids) & ((t_col == tlist[t + tfac]) | (t_col == tlist[pret]))

          # pick up the data that will be used to compute ATT(g,t)
          disdat <- data[disidx, ]
          # positional index into `data` for slicing the global design matrix, kept in
          # sync with any later row drops below (droplevels does not reorder/drop rows).
          # global_mm[disdat_rows, ] is the design for the rows used in this cell.
          disdat_rows <- which(disidx)

          # drop missing factors
          disdat <- droplevels(disdat)

          # give short names for data in this iteration
          G <- disdat$.G
          C <- disdat$.C
          Y <- disdat[[yname]]
          post <- 1 * (disdat[[tname]] == tlist[t + tfac])
          cell_rowid <- disdat$.rowid
        }
        # num obs. for computing ATT(g,t), have to be careful here
        n1 <- sum(G + C)

        # Handle fix_weights for RC/unbalanced panel
        if (!is.null(fix_weights) && fix_weights %in% c("base_period", "first_period")) {
          # Determine which period's weight to use
          if (fix_weights == "base_period") {
            target_idx <- pret_g
          } else {
            target_idx <- 1L
          }
          target_period <- tlist[target_idx]
          # Map each observation's weight from the target period by integer .rowid
          # matching (equivalent to the previous named-character lookup: first match
          # wins, unmatched -> NA), without coercing every .rowid to character.
          if (use_precompute_rc) {
            # per-period lookup vectors precomputed before the loops; the period
            # subsets preserve data order, so the match() result is identical to
            # the legacy full-table subset below.
            w <- target_w_by_period[[target_idx]][match(cell_rowid, target_rowid_by_period[[target_idx]])]
          } else {
            # Build lookup: weight from target period per unit
            target_rows <- data[t_col == target_period, ]
            w <- target_rows$.w[match(cell_rowid, target_rows$.rowid)]
          }
          # Drop units not observed in the target period
          missing_w <- is.na(w)
          if (any(missing_w)) {
            n_dropped <- length(unique(cell_rowid[missing_w]))
            warning(paste0("Dropped ", n_dropped, " units not observed in ",
                           fix_weights, " (period ", target_period, ") ",
                           "for group ", glist[g], " in time period ", tlist[t + tfac]))
            disdat_rows <- disdat_rows[!missing_w]
            cell_rowid <- cell_rowid[!missing_w]
            if (use_precompute_rc) {
              G <- G[!missing_w]
              C <- C[!missing_w]
              Y <- Y[!missing_w]
              post <- post[!missing_w]
            } else {
              disdat <- disdat[!missing_w, ]
              G <- disdat$.G
              C <- disdat$.C
              Y <- disdat[[yname]]
              post <- 1 * (disdat[[tname]] == tlist[t + tfac])
            }
            n1 <- sum(G + C)
            w <- w[!missing_w]
          }
        } else {
          w <- if (use_precompute_rc) w_col_rc[disdat_rows] else disdat$.w
        }

        #-----------------------------------------------------------------------------
        # checks to make sure that we have enough observations
        skip_this_att_gt <- FALSE
        if (sum(G * post) == 0) {
          warning(paste0("No units in group ", glist[g], " in time period ", tlist[t + tfac]))
          skip_this_att_gt <- TRUE
        }
        if (sum(G * (1 - post)) == 0) {
          # the empty period is the base (pre) period tlist[pret], not tlist[t]
          # (they differ under base_period = "universal" and for post-treatment
          # cells under "varying"); matches the fast path's pret_val
          warning(paste0("No units in group ", glist[g], " in time period ", tlist[pret]))
          skip_this_att_gt <- TRUE
        }
        if (sum(C * post) == 0) {
          warning(paste0("No available control units for group ", glist[g], " in time period ", tlist[t + tfac]))
          skip_this_att_gt <- TRUE
        }
        if (sum(C * (1 - post)) == 0) {
          # same base-period indexing fix as the group check above
          warning(paste0("No available control units for group ", glist[g], " in time period ", tlist[pret]))
          skip_this_att_gt <- TRUE
        }

        if (skip_this_att_gt) {
          attgt.list[[counter]] <- list(att = NA, group = glist[g], year = tlist[(t + tfac)], post = post.treat)
          # inffunc[,counter] <- NA
          # counter <- counter+1
          if (do_inf) inffunc_updates[[update_counter]] <- list(
            indices = seq_len(n),
            values = rep(NA_real_, n)
          )

          # Update the counters
          update_counter <- update_counter + 1
          counter <- counter + 1
          next
        }


        # matrix of covariates: row-subset the global design by position.
        covariates <- global_mm[disdat_rows, , drop = FALSE]
        dimnames(covariates) <- list(NULL, colnames(covariates))

        #-----------------------------------------------------------------------------
        # code for actually computing att(g,t)
        #-----------------------------------------------------------------------------

        attgt <- tryCatch({
          # more checks for enough observations in each group; run inside the
          # tryCatch (with the same stop("overlap")/stop("singular") sentinels
          # as the panel branch) so that a failing preliminary fit degrades to
          # an NA cell with a warning instead of aborting the entire att_gt()
          if (!is.function(est_method)) {
            # checks for pscore based methods (no caching on the RC branch: the
            # cohort index varies cell by cell here)
            if (est_method %in% c("dr", "ipw")) {
              if (overlap_check_fail(covariates, G, intercept_only)) {
                warning(paste0("overlap condition violated for group ", glist[g], " in time period ", tlist[t + tfac]))
                stop("overlap")
              }
            }

            # check if can run regression using control units
            if (est_method %in% c("dr", "reg")) {
              if (rcond_check_fail(covariates, G, intercept_only)) {
                warning(paste0("Covariate matrix for control units is singular or numerically ill-conditioned for group ", glist[g], " in time period ", tlist[t + tfac], "; consider centering/rescaling covariates or removing collinear terms"))
                stop("singular")
              }
            }
          }

          if (inherits(est_method, "function")) {
            # user-specified function
            res <- do.call(est_method, c(list(
              y = Y,
              post = post,
              D = G,
              covariates = covariates,
              i.weights = w,
              inffunc = do_inf
            ), extra_args))
          } else if (est_method == "ipw") {
            # inverse-probability weights
            res <- DRDID::std_ipw_did_rc(
              y = Y,
              post = post,
              D = G,
              covariates = covariates,
              i.weights = w,
              boot = FALSE, inffunc = do_inf
            )
          } else if (est_method == "reg") {
            # regression
            res <- DRDID::reg_did_rc(
              y = Y,
              post = post,
              D = G,
              covariates = covariates,
              i.weights = w,
              boot = FALSE, inffunc = do_inf
            )
          } else {
            # doubly robust, this is default
            res <- DRDID::drdid_rc(
              y = Y,
              post = post,
              D = G,
              covariates = covariates,
              i.weights = w,
              boot = FALSE, inffunc = do_inf
            )
          }

          # n/n1 adjusts for estimating the att_gt only using observations from
          # groups G and C (skipped for point estimates only)
          if (do_inf) res$att.inf.func <- (n / n1) * res$att.inf.func

          # If ATT is NaN, replace it with NA, and mark influence function as missing
          if (is.nan(res$ATT)) {
            res$ATT <- NA
            if (do_inf) res$att.inf.func <- rep(NA_real_, length(res$att.inf.func))
          }
          res
        }, error = function(e) {
          # The overlap/rank guards above already emitted a full diagnostic warning
          # before signalling the internal "overlap"/"singular" sentinels, so skip
          # the wrapper warning for those: each failed cell warns exactly once,
          # matching the fast path. Genuine estimator errors are still surfaced.
          if (!(conditionMessage(e) %in% c("overlap", "singular"))) {
            warning("Error computing internal 2x2 DiD for (g, t) = (", glist[g], ", ", tlist[t + tfac], "): ", e$message, ". The ATT for this cell will be set to NA.")
          }
          NULL
        })

        if (is.null(attgt)) {
          attgt.list[[counter]] <- list(att = NA, group = glist[g], year = tlist[(t + tfac)], post = post.treat)
          if (do_inf) inffunc_updates[[update_counter]] <- list(
            indices = seq_len(n),
            values = rep(NA_real_, n)
          )
          update_counter <- update_counter + 1
          counter <- counter + 1
          next
        }
      } # end panel if

      # save results for this att(g,t)
      attgt.list[[counter]] <- list(
        att = attgt$ATT, group = glist[g], year = tlist[(t + tfac)], post = post.treat
      )


      # populate the influence function in the right places (skipped for point
      # estimates only -- compute_inffunc = FALSE -- so we neither aggregate nor
      # store the per-cell influence function)
      if (do_inf) {
      if (panel) {
        # Store integer indices directly (avoids which() later). On the precompute
        # path `kept` is exactly which(disidx) (disidx is not modified in between),
        # so reuse it; the get_wide_data fallback derives the index from its
        # wide-data disidx as before.
        idx <- if (use_precompute_panel) kept else which(disidx)
        inffunc_updates[[update_counter]] <- list(
          indices = idx,
          values = attgt$att.inf.func
        )
      } else {
        # aggregate inf functions by id (order by id)
        # Use the current cell ids (may differ from the initial selection if
        # fix_weights dropped obs). rowsum(reorder = TRUE) sums per id and returns
        # rows in ascending-id order, matching stats::aggregate() but ~40x faster
        # (see test-slowpath-rowsum).
        if (use_precompute_rc) {
          # every row of this cell satisfies .G == 1 | .C == 1 by construction
          # (the positional assembly selects on exactly that condition, and gname
          # is time-invariant within .rowid -- pre_process_did stops otherwise),
          # so the legacy G|C re-filter below is vacuous here. unique_rowid was
          # precomputed before the loops (loop-invariant).
          current_ids <- cell_rowid
          rs <- rowsum(attgt$att.inf.func, current_ids, reorder = TRUE)
          rs_ids <- sort(unique(current_ids))
          idx <- match(rs_ids, unique_rowid)
        } else {
          current_ids <- disdat$.rowid[disdat$.G == 1 | disdat$.C == 1]
          rs <- rowsum(attgt$att.inf.func, current_ids, reorder = TRUE)
          rs_ids <- sort(unique(current_ids))
          idx <- match(rs_ids, unique(data$.rowid))
        }
        inffunc_updates[[update_counter]] <- list(
          indices = idx,
          values = as.numeric(rs[, 1])
        )
      }
      }


      # save it in influence function matrix
      # inffunc[g,t,] <- inf.func
      # inffunc[,counter] <- inf.func
      update_counter <- update_counter + 1

      # update counter
      counter <- counter + 1
    } # end looping over t
  } # end looping over g

  # Build the influence function sparse matrix directly from triplets.
  # Gather per-column indices/values into lists and concatenate once, rather than
  # growing the triplet vectors with c() on every iteration (which reallocates
  # O(ncol^2) in total). The triplet order is unchanged (column-major, original
  # within-column order), so the resulting sparse matrix is identical.
  # Skipped for point estimates only (compute_inffunc = FALSE): no matrix is returned.
  if (do_inf) {
    nz_list <- lapply(inffunc_updates, `[[`, "indices")
    val_list <- lapply(inffunc_updates, `[[`, "values")
    trip_i <- unlist(nz_list, use.names = FALSE)
    trip_x <- unlist(val_list, use.names = FALSE)
    trip_j <- rep.int(seq_along(inffunc_updates), lengths(nz_list))
    inffunc <- Matrix::sparseMatrix(
      i = trip_i,
      j = trip_j,
      x = trip_x,
      dims = c(inffunc_nrow, length(inffunc_updates))
    )
  } else {
    inffunc <- NULL
  }

  return(list(attgt.list = attgt.list, inffunc = inffunc))
}
