From df2df5b4ac62d9d9846b39d3354f9d110f9a78df Mon Sep 17 00:00:00 2001
From: h7x4 <h7x4@nani.wtf>
Date: Thu, 4 Jun 2026 15:49:30 +0900
Subject: [PATCH] kana_transliteration: handle k->h itermark translit, add more
 tests

---
 src/kana_transliteration.rs | 77 +++++++++++++++++++++----------------
 src/lib.rs                  |  1 +
 src/util.rs                 | 21 ++++++++++
 3 files changed, 66 insertions(+), 33 deletions(-)
 create mode 100644 src/util.rs

diff --git a/src/kana_transliteration.rs b/src/kana_transliteration.rs
index 325657c..a01f317 100644
--- a/src/kana_transliteration.rs
+++ b/src/kana_transliteration.rs
@@ -1,24 +1,4 @@
-fn transpose_codepoints_in_range(
-    text: &str,
-    distance: i32,
-    range_start: u32,
-    range_end: u32,
-) -> String {
-    debug_assert!(range_start <= range_end);
-    debug_assert!(i64::from(range_start) + i64::from(distance) >= 0);
-    debug_assert!(i64::from(range_end) + i64::from(distance) <= i64::from(u32::MAX));
-
-    text.chars()
-        .map(|c| {
-            let codepoint = c as u32;
-            if range_start <= codepoint && codepoint <= range_end {
-                std::char::from_u32(codepoint.strict_add_signed(distance)).unwrap_or(c)
-            } else {
-                c
-            }
-        })
-        .collect()
-}
+use crate::util::transpose_codepoints_in_range;
 
 fn hiragana_vowel_map(hiragana: char) -> Option<char> {
     match hiragana {
@@ -132,22 +112,18 @@ fn katakana_vowel_map(katakana: char) -> Option<char> {
 // - Basic Latin: https://www.unicode.org/charts/PDF/U0000.pdf
 // - Hiragana: https://www.unicode.org/charts/PDF/U3040.pdf
 // - Katakana: https://www.unicode.org/charts/PDF/U30A0.pdf
-// - Halfwidth characters: https://www.unicode.org/charts/PDF/UFF00.pdf
+// - Halfwidth and Fullwidth Forms: https://www.unicode.org/charts/PDF/UFF00.pdf
 
 pub fn transliterate_katakana_to_hiragana(katakana: &str) -> String {
-    let first_pass = transpose_codepoints_in_range(katakana, -96, 12449, 12534);
-
-    // Find instances of long vowel marks (ー), and convert them into long vowels.
+    let first_pass = transpose_codepoints_in_range(katakana, -96, 'ア'.into(), 'ヶ'.into());
 
     // TODO: don't create a new result, just mutate the first pass in-place.
     let chars = first_pass.chars().collect::<Vec<_>>();
     let mut result = String::with_capacity(chars.len());
 
     for (i, c) in chars.iter().enumerate() {
-        if *c == 'ー' {
-            if i == 0 {
-                result.push('ー');
-            } else {
+        match *c {
+            'ー' if i > 0 => {
                 let prev_char = result.chars().last().unwrap();
                 if let Some(vowel) = hiragana_vowel_map(prev_char) {
                     result.push(vowel);
@@ -155,8 +131,9 @@ pub fn transliterate_katakana_to_hiragana(katakana: &str) -> String {
                     result.push('ー');
                 }
             }
-        } else {
-            result.push(*c);
+            'ヽ' if i > 0 => result.push('ゝ'),
+            'ヾ' if i > 0 => result.push('ゞ'),
+            _ => result.push(*c),
         }
     }
 
@@ -186,9 +163,9 @@ pub fn transliterate_hiragana_to_katakana(hiragana: &str) -> String {
     result
 }
 
-pub fn transliterate_fullwidth_romaji_to_halfwidth_romaji(halfwidth: &str) -> String {
+pub fn transliterate_fullwidth_romaji_to_halfwidth_romaji(fullwidth: &str) -> String {
     transpose_codepoints_in_range(
-        &transpose_codepoints_in_range(halfwidth, -65248, 0xFF01, 0xFF5E),
+        &transpose_codepoints_in_range(fullwidth, -65248, 0xFF01, 0xFF5E),
         -12256,
         12288,
         12288,
@@ -502,6 +479,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_katakana_to_hiragana_dont_convert_standalone_dash() {
+        assert_eq!(
+            transliterate_katakana_to_hiragana("ーかたかな"),
+            "ーかたかな"
+        );
+    }
+
+    #[test]
+    fn test_katakana_to_hiragana_iteration_markers() {
+        assert_eq!(transliterate_katakana_to_hiragana("コヽロ"), "こゝろ");
+    }
+
+    #[test]
+    fn test_katakana_to_hiragana_iteration_markers_dakuten() {
+        assert_eq!(transliterate_katakana_to_hiragana("サヾエ"), "さゞえ");
+    }
+
+    #[ignore = "FIX ME"]
+    #[test]
+    fn test_katakana_to_hiragana_dash_following_iteration_markers() {
+        assert_eq!(transliterate_katakana_to_hiragana("サヾーエ"), "さゞあえ");
+    }
+
+    #[test]
+    fn test_katakana_to_hiragana_wi_we() {
+        assert_eq!(transliterate_katakana_to_hiragana("ヰヱ"), "ゐゑ");
+    }
+
     #[test]
     fn test_hiragana_to_katakana() {
         assert_eq!(transliterate_hiragana_to_katakana("ひらがな"), "ヒラガナ");
@@ -523,6 +529,11 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_hiragana_to_katakana_wi_we() {
+        assert_eq!(transliterate_hiragana_to_katakana("ゐゑ"), "ヰヱ");
+    }
+
     #[test]
     fn test_fullwidth_romaji_to_halfwidth_romaji() {
         assert_eq!(
diff --git a/src/lib.rs b/src/lib.rs
index 1e6f3d3..7705a3c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,6 @@
 mod kana_transliteration;
 mod text_normalization;
+mod util;
 
 use sqlite3_ext::{
     Connection, FromValue, Result, ValueRef, ValueType, function::Context, sqlite3_ext_fn,
diff --git a/src/util.rs b/src/util.rs
new file mode 100644
index 0000000..60df67a
--- /dev/null
+++ b/src/util.rs
@@ -0,0 +1,21 @@
+pub(crate) fn transpose_codepoints_in_range(
+    text: &str,
+    distance: i32,
+    range_start: u32,
+    range_end: u32,
+) -> String {
+    debug_assert!(range_start <= range_end);
+    debug_assert!(i64::from(range_start) + i64::from(distance) >= 0);
+    debug_assert!(i64::from(range_end) + i64::from(distance) <= i64::from(u32::MAX));
+
+    text.chars()
+        .map(|c| {
+            let codepoint = c as u32;
+            if range_start <= codepoint && codepoint <= range_end {
+                std::char::from_u32(codepoint.strict_add_signed(distance)).unwrap_or(c)
+            } else {
+                c
+            }
+        })
+        .collect()
+}