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perf(gsub): ScriptList/FeatureList 整块拷贝快路径（同 GPOS）

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subsetGSUB 复用 scriptListSpan/featureListSpan，span 自洽且不越过相邻 list
header offset 时整块 subarray 拷贝。交错判定用 SL span≤featureListOff、
FL span≤lookupListOff（LookupList subtable 散落，header offset 不能代表其
字节范围，故不用「下一更大 offset」作上界）。霞鹜文楷 GSUB 物理交错
(LL<SL<FL 且 ScriptTable 跨越 FL) 自动降级 serialize，字节与优化前完全一致。

收益：初夏 GSUB 11474→8474B（-26%）、FiraCode 15504→14878B（-4%），
保留 fontTools FeatureTable 去重。霞鹜降级路径 cmp 逐字节一致。全基准 SSIM 不变。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
崮生（子虚） 2026-07-24 11:23:06 +08:00
parent ccc2e995c8
commit 3103dd5db1
1 changed files with 23 additions and 3 deletions
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26
backend/font_util/gsub-subset.ts
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@ -21,7 +21,7 @@
* @reference https://learn.microsoft.com/en-us/typography/opentype/spec/gsub
*/
import { OTWriter as Writer, OTReader as Reader, serializeScriptList, serializeFeatureList } from "./ot-bytes.js";
import { OTWriter as Writer, OTReader as Reader, serializeScriptList, serializeFeatureList, scriptListSpan, featureListSpan } from "./ot-bytes.js";
/** GSUB lookup 类型常量 */
const LT_SINGLE = 1;
@ -1252,8 +1252,28 @@ export function subsetGSUB(
* - 支持的：gid 重映射后重新序列化
* - 不支持的（type5 等）：原样拷贝原始字节（gid 不重映射，浏览器查 coverage 查不到会跳过）
*/
const scriptListBytes = serializeScriptList(r, scriptListOff);
const featureListBytes = serializeFeatureList(r, featureListOff);
/** ScriptList / FeatureList 整块拷贝快路径（同 subsetGPOS）：
* 两表不含 glyphId，若子表紧凑排列不与相邻 list 物理交错，字节块本身即合法表，
* 直接 subarray 拷贝跳过逐字段序列化，并保留 fontTools 去重。
*
* 交错判定用「span 不越过下一 list 的 header offset」：SL 上界为 featureListOff，
* FL 上界为 lookupListOff。霞鹜文楷 GSUB 布局为 LookupList(10) < ScriptList(38)
* < FeatureList(76)，ScriptTable 跨越 FeatureList 起始、且 LookupList 数据散落在
* FeatureList 之后——此时 lookupListOff(10) < featureListOff(76)，FL 的 span 必然
* > lookupListOff 而降级；SL span(350) 也越过 fl(76) 而降级。故物理交错字体自动走
* serialize，安全。不使用「三个 offset 中下一个更大值」作上界：LookupList 的 subtable
* 可散落在任意偏移，header offset 不能代表其字节范围。 */
const slSpan = scriptListSpan(r, scriptListOff);
const slContiguous = slSpan >= 0 && scriptListOff + slSpan <= featureListOff;
const scriptListBytes = slContiguous
? gsubBytes.subarray(scriptListOff, scriptListOff + slSpan)
: serializeScriptList(r, scriptListOff);
const flSpan = featureListSpan(r, featureListOff);
const flContiguous = flSpan >= 0 && featureListOff + flSpan <= lookupListOff;
const featureListBytes = flContiguous
? gsubBytes.subarray(featureListOff, featureListOff + flSpan)
: serializeFeatureList(r, featureListOff);
r.clearError();
/** ScriptList/FeatureList 解析失败（异常表）则整体保留原始 GSUB 字节（安全降级） */
if (!scriptListBytes || !featureListBytes) return gsubBytes;