From 2b7e96835e75e0d153e7f993d1c4fc2add452ddd Mon Sep 17 00:00:00 2001
From: Bruce Hill <bruce@bruce-hill.com>
Date: Sun, 8 Feb 2026 22:47:02 -0500
Subject: Added Text.distance(a,b) for text similarity comparisons.

---
 src/stdlib/text.c | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/stdlib/text.h |  1 +
 2 files changed, 65 insertions(+)

(limited to 'src/stdlib')

diff --git a/src/stdlib/text.c b/src/stdlib/text.c
index 4bf6d999..117b4a8d 100644
--- a/src/stdlib/text.c
+++ b/src/stdlib/text.c
@@ -1426,6 +1426,70 @@ Text_t Text$title(Text_t text, Text_t language) {
     return ret;
 }
 
+public
+double Text$distance(Text_t a, Text_t b, Text_t language) {
+    if (a.length == 0) return (double)b.length;
+    if (b.length == 0) return (double)a.length;
+
+    // The current implementation of text distance uses a modified form
+    // of Damerau–Levenshtein distance that gives slightly lower distances
+    // for letters with the same main grapheme and slightly lower distances
+    // for letters that are the same, but with different casing.
+    double *distances = GC_MALLOC_ATOMIC(sizeof(uint32_t[a.length][b.length]));
+#define DIST(x, y) distances[(x) * b.length + (y)]
+    for (int64_t i = 0; i <= a.length; i++)
+        DIST(i, 0) = i;
+    for (int64_t j = 0; j <= b.length; j++)
+        DIST(0, j) = j;
+
+    TextIter_t a_state = NEW_TEXT_ITER_STATE(a);
+    TextIter_t b_state = NEW_TEXT_ITER_STATE(b);
+    const char *uc_language = Text$as_c_string(language);
+    for (int64_t i = 1; i <= a.length; i++) {
+        for (int64_t j = 1; j <= b.length; j++) {
+            int32_t ai = Text$get_grapheme_fast(&a_state, i - 1);
+            int32_t bi = Text$get_grapheme_fast(&b_state, i - 1);
+            if (ai == bi) {
+                DIST(i, j) = DIST(i - 1, j - 1);
+            } else {
+                ucs4_t main_ai = (ai) >= 0 ? (ucs4_t)ai : synthetic_graphemes[-(ai)-1].main_codepoint;
+                ucs4_t main_bi = (bi) >= 0 ? (ucs4_t)bi : synthetic_graphemes[-(bi)-1].main_codepoint;
+                if (main_ai == main_bi) {
+                    // Same main grapheme (different modifiers)
+                    DIST(i, j) = 0.25 + DIST(i - 1, j - 1);
+                } else {
+                    int cmp;
+                    (void)u32_casecmp(&main_ai, 1, &main_bi, 1, uc_language, UNINORM_NFC, &cmp);
+                    if (cmp == 0) {
+                        // Same main grapheme, different casing (e.g. "a" vs "A")
+                        DIST(i, j) = 0.5 + DIST(i - 1, j - 1);
+                    } else {
+                        // Different main grapheme
+                        double insertion = 1. + DIST(i - 1, j);
+                        double deletion = 1. + DIST(i, j - 1);
+                        double dist = MIN(insertion, deletion);
+                        double substitution = 1. + DIST(i - 1, j - 1);
+                        dist = MIN(dist, substitution);
+                        // Check for transposition:
+                        if (i >= 2 && j >= 2) {
+                            int32_t ai_prev = Text$get_grapheme_fast(&a_state, i - 2);
+                            int32_t bi_prev = Text$get_grapheme_fast(&b_state, i - 2);
+                            if (ai == bi_prev && bi == ai_prev) {
+                                double transposition = 1. + DIST(i - 2, j - 2);
+                                dist = MIN(dist, transposition);
+                            }
+                        }
+                        DIST(i, j) = dist;
+                    }
+                }
+            }
+        }
+    }
+#undef DIST
+
+    return (double)distances[a.length * b.length + b.length];
+}
+
 public
 Text_t Text$escaped(Text_t text, bool colorize, Text_t extra_escapes) {
     Text_t ret = colorize ? Text("\x1b[35m") : EMPTY_TEXT;
diff --git a/src/stdlib/text.h b/src/stdlib/text.h
index 856b173a..7259b545 100644
--- a/src/stdlib/text.h
+++ b/src/stdlib/text.h
@@ -116,6 +116,7 @@ Int_t Text$width(Text_t text, Text_t language);
 Text_t Text$left_pad(Text_t text, Int_t width, Text_t padding, Text_t language);
 Text_t Text$right_pad(Text_t text, Int_t width, Text_t padding, Text_t language);
 Text_t Text$middle_pad(Text_t text, Int_t width, Text_t padding, Text_t language);
+double Text$distance(Text_t a, Text_t b, Text_t language);
 int32_t Text$get_grapheme_fast(TextIter_t *state, int64_t index);
 uint32_t Text$get_main_grapheme_fast(TextIter_t *state, int64_t index);
 Int_t Text$memory_size(Text_t text);
-- 
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