From ed88711c10346207449881c261ed1a4287067470 Mon Sep 17 00:00:00 2001
From: Alexander Shtuchkin <ashtuchkin@gmail.com>
Date: Sun, 23 May 2021 18:32:46 -0400
Subject: [PATCH] Fix Big5-HKSCS encoding to prefer non-HKSCS codes in case of
 multiple options (fixes #264)

---
 encodings/dbcs-data.js | 14 +++++++++++++-
 generation/gen-dbcs.js | 32 ++++++++++++++++++++++++++++++--
 test/big5-test.js      |  4 ++++
 3 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/encodings/dbcs-data.js b/encodings/dbcs-data.js
index 4b61914..0d17e58 100644
--- a/encodings/dbcs-data.js
+++ b/encodings/dbcs-data.js
@@ -167,7 +167,19 @@ module.exports = {
     'big5hkscs': {
         type: '_dbcs',
         table: function() { return require('./tables/cp950.json').concat(require('./tables/big5-added.json')) },
-        encodeSkipVals: [0xa2cc],
+        encodeSkipVals: [
+            // Although Encoding Standard says we should avoid encoding to HKSCS area (See Step 1 of
+            // https://encoding.spec.whatwg.org/#index-big5-pointer), we still do it to increase compatibility with ICU.
+            // But if a single unicode point can be encoded both as HKSCS and regular Big5, we prefer the latter.
+            0x8e69, 0x8e6f, 0x8e7e, 0x8eab, 0x8eb4, 0x8ecd, 0x8ed0, 0x8f57, 0x8f69, 0x8f6e, 0x8fcb, 0x8ffe,
+            0x906d, 0x907a, 0x90c4, 0x90dc, 0x90f1, 0x91bf, 0x92af, 0x92b0, 0x92b1, 0x92b2, 0x92d1, 0x9447, 0x94ca,
+            0x95d9, 0x96fc, 0x9975, 0x9b76, 0x9b78, 0x9b7b, 0x9bc6, 0x9bde, 0x9bec, 0x9bf6, 0x9c42, 0x9c53, 0x9c62,
+            0x9c68, 0x9c6b, 0x9c77, 0x9cbc, 0x9cbd, 0x9cd0, 0x9d57, 0x9d5a, 0x9dc4, 0x9def, 0x9dfb, 0x9ea9, 0x9eef,
+            0x9efd, 0x9f60, 0x9fcb, 0xa077, 0xa0dc, 0xa0df, 0x8fcc, 0x92c8, 0x9644, 0x96ed,
+
+            // Step 2 of https://encoding.spec.whatwg.org/#index-big5-pointer: Use last pointer for U+2550, U+255E, U+2561, U+256A, U+5341, or U+5345
+            0xa2a4, 0xa2a5, 0xa2a7, 0xa2a6, 0xa2cc, 0xa2ce,
+        ],
     },
 
     'cnbig5': 'big5hkscs',
diff --git a/generation/gen-dbcs.js b/generation/gen-dbcs.js
index 6c877ed..a3d550d 100644
--- a/generation/gen-dbcs.js
+++ b/generation/gen-dbcs.js
@@ -27,7 +27,7 @@ async.parallel({
     }
 
     // Calculate difference between big5 and cp950, and write it to a file.
-    // See http://encoding.spec.whatwg.org/#big5-encoder
+    // See http://encoding.spec.whatwg.org/#big5
     var big5add = {}
     for (var i = 0x8100; i < 0x10000; i++) { // Lead byte is 0x81 .. 0xFE
         var trail = i & 0xFF;
@@ -41,7 +41,35 @@ async.parallel({
             big5add[i] = big5Char;
     }
 
-    // Add char sequences that are not in the index file (as given in http://encoding.spec.whatwg.org/#big5-encoder)
+    // Calculate HKSCS codes that are duplicates of big5 codes and need to be skipped when encoding.
+    console.log("Duplicate HKSCS codes that need to be skipped when encoded (see encodeSkipVals in big5hkscs): ")
+    var big5codes = {};
+    for (var i = 0xA100; i < 0x10000; i++) {
+        var uCharCode = (big5add[i] !== undefined) ? big5add[i] : data.cp950[i];
+        if (uCharCode !== undefined) {
+            big5codes[uCharCode] = true;
+        }
+    }
+    for (var i = 0x8100; i < 0xA100; i++) {
+        var uCharCode = (big5add[i] !== undefined) ? big5add[i] : data.cp950[i];
+        if (uCharCode !== undefined && big5codes[uCharCode]) {
+            console.log("0x"+i.toString(16));
+        }
+    }
+
+    if (big5Char !== undefined) {
+        if (lead < 0xA1) {
+            if (d[big5Char] !== undefined) {
+                console.log("duplicate in first: "+ pointer + " char " + big5Char);
+            }
+            d[big5Char] = i;
+        } else if (d[big5Char] !== undefined) {
+            console.log("dup 0x"+d[big5Char].toString(16) + " -> " + i.toString(16))
+        }
+
+    }
+
+    // Add char sequences that are not in the index file (as given in http://encoding.spec.whatwg.org/#big5-decoder)
     function toIdx(pointer) { var trail = pointer % 157; var lead = Math.floor(pointer / 157) + 0x81; return (lead << 8) + (trail + (trail < 0x3F ? 0x40 : 0x62))}
     big5add[toIdx(1133)] = [0x00CA, 0x0304];
     big5add[toIdx(1135)] = [0x00CA, 0x030C];
diff --git a/test/big5-test.js b/test/big5-test.js
index 839a3f6..1ca9dd9 100644
--- a/test/big5-test.js
+++ b/test/big5-test.js
@@ -54,4 +54,8 @@ describe("Big5 tests", function() {
     it("Big5 correctly encodes 十", function() {
         assert.strictEqual(iconv.encode("十", "big5").toString('hex'), "a451");
     });
+
+    it("Big5 correctly encodes 起 (issue #264)", function() {
+        assert.strictEqual(iconv.encode("起", "big5").toString('hex'), "b05f");
+    });
 });