Use meaningful group names in dbind()

When a named list of ISMs is passed to dbind(), use the list names as
group levels in the resulting BISM. When BISMs are in the mix, preserve
their existing @groups levels. Unnamed entries fall back to position-based
numeric indices, skipping any that collide with existing labels.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bucknerj

R/InfinitySparseMatrix.R

@@ -973,34 +973,61 @@ dbind <- function(..., force_unique_names = FALSE) {
 
 
   # Convert all matrices to ISMs if they aren't already.
-  mats <- lapply(mats, function(x) {
+  # Also build a parallel vector of group labels for each entry.
+  input_names <- names(mats)
+  converted <- lapply(seq_along(mats), function(i) {
+    x <- mats[[i]]
+    nm <- if (!is.null(input_names) && nzchar(input_names[i])) input_names[i] else NA_character_
+
     if (is(x, "BlockedInfinitySparseMatrix")) {
-      # Replace BISM with list of ISMs
-      findSubproblems(x)
+      # Replace BISM with list of ISMs; use its existing group level names
+      sp <- findSubproblems(x)
+      list(mats = sp, labels = names(sp))
     } else if (inherits(x, "list")) {
       # If any entry in ... is a list,
+      inner_names <- names(x)
       # 1) Convert all entries in that list to ISM while keeping BISM as BISM
       x <- lapply(x, .as.ism_or_bism)
-      # 2) If we have any BISMs, split into list of ISMS
-      x <- lapply(x, function(y) {
+      # 2) If we have any BISMs, split into list of ISMs, preserving labels
+      inner_converted <- lapply(seq_along(x), function(j) {
+        y <- x[[j]]
+        inner_nm <- if (!is.null(inner_names) && nzchar(inner_names[j])) inner_names[j] else NA_character_
         if (is(y, "BlockedInfinitySparseMatrix")) {
-          findSubproblems(y)
+          sp <- findSubproblems(y)
+          list(mats = sp, labels = names(sp))
         } else {
-          y
+          list(mats = y, labels = inner_nm)
         }
       })
       # 3) pull list of lists into list
-      flatten_list(x)
+      list(mats = flatten_list(lapply(inner_converted, `[[`, "mats")),
+           labels = unlist(lapply(inner_converted, `[[`, "labels")))
     } else {
       # This will error appropriately if some element in `mats` cannot be
       # converted to an ISM.
-      .as.ism_or_bism(x)
+      list(mats = .as.ism_or_bism(x), labels = nm)
     }
   })
 
   # If we were passed any BISMs, we have a list of lists of ISM, so flatten to a
   # single list.
-  mats <- flatten_list(mats)
+  mats <- flatten_list(lapply(converted, `[[`, "mats"))
+  group_labels <- unlist(lapply(converted, `[[`, "labels"))
+
+  # Replace NA labels (from unnamed entries) with numeric indices based on
+  # their position, incrementing to avoid collisions with existing labels.
+  na_idx <- which(is.na(group_labels))
+  if (length(na_idx) > 0) {
+    existing <- group_labels[!is.na(group_labels)]
+    for (i in na_idx) {
+      candidate <- i
+      while (as.character(candidate) %in% existing) {
+        candidate <- candidate + 1L
+      }
+      group_labels[i] <- as.character(candidate)
+      existing <- c(existing, as.character(candidate))
+    }
+  }
 
   # new row and column positions are based on current, incrementing by number of
   # rows/columns in all previous matrices.
@@ -1052,10 +1079,10 @@ dbind <- function(..., force_unique_names = FALSE) {
   newdim <- as.integer(c(sum(vapply(lapply(mats, methods::slot, "dimension"), "[", 1, 1)),
                          sum(vapply(lapply(mats, methods::slot, "dimension"), "[", 1, 2))))
 
-  # This needs to be much smarter, especially if any element is already a BISM
-  groups <- as.factor(rep(seq_along(mats), times =
+  groups <- factor(rep(group_labels, times =
                                       vapply(lapply(mats, slot, "colnames"), length, 1) +
-                                        vapply(lapply(mats, slot, "rownames"), length, 1)))
+                                        vapply(lapply(mats, slot, "rownames"), length, 1)),
+                   levels = unique(group_labels))
   names(groups) <- do.call(c, Map(c, cnameslist, rnameslist))
 
   newdata <- do.call(c, mats)

tests/testthat/test.InfinitySparseMatrix.R

@@ -955,6 +955,46 @@ test_that("dbind", {
   expect_identical(bmix1, bmix3)
 })
 
+test_that("dbind uses meaningful group names", {
+  data(nuclearplants)
+  np <- nuclearplants
+  np$group <- as.numeric(cut(np$cap, breaks = c(0, 600, 825, 1000, 2000)))
+
+  m1 <- match_on(pr ~ cost, data = np[np$group == 1,])
+  m2 <- match_on(pr ~ cost, data = np[np$group == 2,])
+  m3 <- match_on(pr ~ cost, data = np[np$group == 3,])
+  m4 <- match_on(pr ~ cost, data = np[np$group == 4,])
+
+  # Named list of ISMs -> group levels match the list names
+  bm_named <- dbind(first = m1, second = m2)
+  expect_identical(levels(bm_named@groups), c("first", "second"))
+
+  # Unnamed list -> levels are "1", "2", ... (backward compat)
+  bm_unnamed <- dbind(m1, m2)
+  expect_identical(levels(bm_unnamed@groups), c("1", "2"))
+
+  # Mixed named list with a BISM entry -> ISM entries get list names,
+  # BISM entries get their original @groups levels
+  b1 <- match_on(pr ~ cost + strata(group), data = np[np$group < 3,])
+  bm_mixed <- dbind(b1, extra = m3)
+  bism_levels <- levels(b1@groups)
+  expect_identical(levels(bm_mixed@groups), c(bism_levels, "extra"))
+
+  # Partially named list -> named entries use names, unnamed fall back to index
+  bm_partial <- dbind(a = m1, m2, m3)
+  expect_identical(levels(bm_partial@groups), c("a", "2", "3"))
+
+  # Unnamed fallback indices skip names that collide with existing labels
+  bm_collision <- dbind(m1, b1)
+  # m1 at position 1 collides with b1's groups "1"/"2", so gets "3"
+  # Levels preserve input order: m1's group first, then b1's groups
+  expect_identical(levels(bm_collision@groups), c("3", bism_levels))
+
+  # Named list passed as single argument
+  bm_named_list <- dbind(list(x = m1, y = m2))
+  expect_identical(levels(bm_named_list@groups), c("x", "y"))
+})
+
 test_that("dbind'ing a very large number of matrices", {
   data(nuclearplants)