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Chris Tate
ba8849a269
`s.charAt(i) === "c"`, and `s[i] === "c"` where the index is proven in bounds, built a one-character string to compare with the literal. When the literal is one UTF-16 code unit, the comparison now reads `s.charCodeAt(i)` and compares the number. charCodeAt converts the index the same way charAt does, and an index out of range answers NaN, which equals no unit, as "" equals no one-unit literal. A switch on `s.charAt(i)` whose case tests are all one-unit literals switches on the code unit the same way. A switch on a `string | undefined` value with string literal cases, such as an array element or an optional field, lowered to a chain of union equalities. It now tests for undefined, which matches no case, and dispatches the string through the string switch: by byte length, then by the literal's leading bytes. `String.fromCharCode(a, b, ...)` with up to sixteen arguments passes the codes to the runtime in a stack array instead of a heap f64[], and up to 64 code units encode into a stack buffer instead of a heap scratch block. `.length` of a string that is not ASCII counts its UTF-16 units. The string index keeps four entries, so a program that read the lengths of more long texts than that in turn (the text of every source file) walked each whole string on every read. Lengths of non-ASCII strings longer than 64 bytes now go into a 1024-slot table keyed by address, allocated by the first length it records. A slot is dropped when the string is freed or reallocated and when an append changes it in place, which is where the index drops its facts.