LibTextCodec: Implement big5 encoder

Implements the `big5` encoder, as specified by
https://encoding.spec.whatwg.org/#big5-encoder

(cherry picked from commit 34c8c559c112796af0f99b48a7b88cb26633a764)

Tests/LibTextCodec/TestTextEncoders.cpp

@@ -61,3 +61,21 @@ TEST_CASE(test_euc_kr_encoder)
     EXPECT(processed_bytes[2] == 0xF0);
     EXPECT(processed_bytes[3] == 0xD8);
 }
+
+TEST_CASE(test_big5_encoder)
+{
+    TextCodec::Big5Encoder encoder;
+    // U+A7 Section Sign
+    // U+70D7 CJK Unified Ideograph-70D7
+    auto test_string = "\U000000A7\U000070D7"sv;
+
+    Vector<u8> processed_bytes;
+    MUST(encoder.process(Utf8View(test_string), [&](u8 byte) {
+        return processed_bytes.try_append(byte);
+    }));
+    EXPECT(processed_bytes.size() == 4);
+    EXPECT(processed_bytes[0] == 0xA1);
+    EXPECT(processed_bytes[1] == 0xB1);
+    EXPECT(processed_bytes[2] == 0xD2);
+    EXPECT(processed_bytes[3] == 0x71);
+}

Userland/Libraries/LibTextCodec/Encoder.cpp

@@ -14,6 +14,7 @@ namespace TextCodec {
 
 namespace {
 UTF8Encoder s_utf8_encoder;
+Big5Encoder s_big5_encoder;
 EUCJPEncoder s_euc_jp_encoder;
 EUCKREncoder s_euc_kr_encoder;
 }
@@ -22,6 +23,8 @@ Optional<Encoder&> encoder_for_exact_name(StringView encoding)
 {
     if (encoding.equals_ignoring_ascii_case("utf-8"sv))
         return s_utf8_encoder;
+    if (encoding.equals_ignoring_ascii_case("big5"sv))
+        return s_big5_encoder;
     if (encoding.equals_ignoring_ascii_case("euc-jp"sv))
         return s_euc_jp_encoder;
     if (encoding.equals_ignoring_ascii_case("euc-kr"sv))
@@ -138,4 +141,70 @@ ErrorOr<void> EUCKREncoder::process(Utf8View input, Function<ErrorOr<void>(u8)>
     return {};
 }
 
+// https://encoding.spec.whatwg.org/#index-big5-pointer
+static Optional<u32> index_big5_pointer(u32 code_point)
+{
+    // 1. Let index be index Big5 excluding all entries whose pointer is less than (0xA1 - 0x81) × 157.
+    auto start_index = (0xA1 - 0x81) * 157 - s_big5_index_first_pointer;
+
+    // 2. If code point is U+2550, U+255E, U+2561, U+256A, U+5341, or U+5345, return the last pointer
+    //    corresponding to code point in index.
+    if (Array<u32, 6> { 0x2550, 0x255E, 0x2561, 0x256A, 0x5341, 0x5345 }.contains_slow(code_point)) {
+        for (u32 i = s_big5_index.size() - 1; i >= start_index; --i) {
+            if (s_big5_index[i] == code_point) {
+                return s_big5_index_first_pointer + i;
+            }
+        }
+        return {};
+    }
+
+    // 3. Return the index pointer for code point in index.
+    for (u32 i = start_index; i < s_big5_index.size(); ++i) {
+        if (s_big5_index[i] == code_point) {
+            return s_big5_index_first_pointer + i;
+        }
+    }
+    return {};
+}
+
+// https://encoding.spec.whatwg.org/#big5-encoder
+ErrorOr<void> Big5Encoder::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte)
+{
+    for (u32 item : input) {
+        // 1. If code point is end-of-queue, return finished.
+
+        // 2. If code point is an ASCII code point, return a byte whose value is code point.
+        if (is_ascii(item)) {
+            TRY(on_byte(static_cast<u8>(item)));
+            continue;
+        }
+
+        // 3. Let pointer be the index Big5 pointer for code point.
+        auto pointer = index_big5_pointer(item);
+
+        // 4. If pointer is null, return error with code point.
+        if (!pointer.has_value()) {
+            // TODO: Report error.
+            continue;
+        }
+
+        // 5. Let lead be pointer / 157 + 0x81.
+        auto lead = *pointer / 157 + 0x81;
+
+        // 6. Let trail be pointer % 157.
+        auto trail = *pointer % 157;
+
+        // 7. Let offset be 0x40 if trail is less than 0x3F, otherwise 0x62.
+        auto offset = 0x62;
+        if (trail < 0x3f)
+            offset = 0x40;
+
+        // 8. Return two bytes whose values are lead and trail + offset.
+        TRY(on_byte(static_cast<u8>(lead)));
+        TRY(on_byte(static_cast<u8>(trail + offset)));
+    }
+
+    return {};
+}
+
 }

Userland/Libraries/LibTextCodec/Encoder.h

@@ -34,6 +34,11 @@ public:
     virtual ErrorOr<void> process(Utf8View, Function<ErrorOr<void>(u8)> on_byte) override;
 };
 
+class Big5Encoder final : public Encoder {
+public:
+    virtual ErrorOr<void> process(Utf8View, Function<ErrorOr<void>(u8)> on_byte) override;
+};
+
 Optional<Encoder&> encoder_for_exact_name(StringView encoding);
 Optional<Encoder&> encoder_for(StringView label);