feat: support alternate encodings

Closes #37

src/Encoding.zig

@@ -0,0 +1,72 @@
+const std = @import("std");
+
+context: *const anyopaque,
+guessFn: *const fn (context: *const anyopaque, text: []const u8) bool,
+checkEncodingFn: *const fn (context: *const anyopaque, xml_encoding: []const u8) bool,
+transcodeFn: *const fn (context: *const anyopaque, noalias dest: []u8, noalias src: []const u8) TranscodeResult,
+
+const Encoding = @This();
+
+pub const TranscodeResult = struct {
+    dest_written: usize,
+    src_read: usize,
+};
+
+pub fn guess(encoding: Encoding, text: []const u8) bool {
+    return encoding.guessFn(encoding.context, text);
+}
+
+pub fn checkEncoding(encoding: Encoding, xml_encoding: []const u8) bool {
+    return encoding.checkEncodingFn(encoding.context, xml_encoding);
+}
+
+pub fn transcode(encoding: Encoding, noalias dest: []u8, noalias src: []const u8) TranscodeResult {
+    return encoding.transcodeFn(encoding.context, dest, src);
+}
+
+pub const utf8: Encoding = .{
+    .context = undefined,
+    .guessFn = &utf8Guess,
+    .checkEncodingFn = &utf8CheckEncoding,
+    .transcodeFn = &utf8Transcode,
+};
+
+fn utf8Guess(context: *const anyopaque, text: []const u8) bool {
+    _ = context;
+    _ = text;
+    return true;
+}
+
+fn utf8CheckEncoding(context: *const anyopaque, encoding: []const u8) bool {
+    _ = context;
+    return std.ascii.eqlIgnoreCase(encoding, "UTF-8");
+}
+
+fn utf8Transcode(context: *const anyopaque, noalias dest: []u8, noalias src: []const u8) TranscodeResult {
+    _ = context;
+    var dest_written: usize = 0;
+    var src_read: usize = 0;
+    while (src_read < src.len) {
+        const cp_len = std.unicode.utf8ByteSequenceLength(src[src_read]) catch break;
+        if (src_read + cp_len > src.len or dest_written + cp_len > dest.len) break;
+        switch (cp_len) {
+            1 => {
+                dest[dest_written] = src[src_read];
+                dest_written += 1;
+                src_read += 1;
+            },
+            2, 3, 4 => {
+                const slice = src[src_read..][0..cp_len];
+                if (!std.unicode.utf8ValidateSlice(slice)) break;
+                @memcpy(dest[dest_written..][0..cp_len], slice);
+                dest_written += cp_len;
+                src_read += cp_len;
+            },
+            else => unreachable,
+        }
+    }
+    return .{
+        .dest_written = dest_written,
+        .src_read = src_read,
+    };
+}

src/Reader.zig

@@ -155,17 +155,22 @@ pub const ErrorCode = enum {
     expected_equals,
     expected_quote,
     missing_end_quote,
-    invalid_utf8,
+    invalid_encoding,
     illegal_character,
 };
 
 pub const Source = struct {
     context: *const anyopaque,
     moveFn: *const fn (context: *const anyopaque, advance: usize, len: usize) anyerror![]const u8,
+    checkEncodingFn: *const fn (context: *const anyopaque, encoding: []const u8) bool,
 
     pub fn move(source: Source, advance: usize, len: usize) anyerror![]const u8 {
         return source.moveFn(source.context, advance, len);
     }
+
+    pub fn checkEncoding(source: Source, encoding: []const u8) bool {
+        return source.checkEncodingFn(source.context, encoding);
+    }
 };
 
 const State = enum {
@@ -1565,7 +1570,7 @@ fn checkXmlVersion(reader: *Reader, version: []const u8, n_attr: usize) !void {
 }
 
 fn checkXmlEncoding(reader: *Reader, encoding: []const u8, n_attr: usize) !void {
-    if (!std.ascii.eqlIgnoreCase(encoding, "utf-8")) {
+    if (!reader.source.checkEncoding(encoding)) {
         return reader.fatal(.xml_declaration_encoding_unsupported, reader.attributeValuePos(n_attr));
     }
 }
@@ -2149,7 +2154,7 @@ fn fatalInvalidUtf8(reader: *Reader, s: []const u8, pos: usize) error{MalformedX
         if (!std.unicode.utf8ValidateSlice(s[invalid_pos..][0..cp_len])) break;
         invalid_pos += cp_len;
     }
-    return reader.fatal(.invalid_utf8, pos + invalid_pos);
+    return reader.fatal(.invalid_encoding, pos + invalid_pos);
 }
 
 const base_read_size = 4096;

src/xml.zig

@@ -77,6 +77,8 @@ pub const predefined_namespace_uris = std.StaticStringMap([]const u8).initCompti
     .{ "xmlns", ns_xmlns },
 });
 
+pub const Encoding = @import("Encoding.zig");
+
 pub const Reader = @import("Reader.zig");
 
 pub fn GenericReader(comptime SourceError: type) type {
@@ -324,6 +326,7 @@ pub const StaticDocument = struct {
         return .{
             .context = doc,
             .moveFn = &move,
+            .checkEncodingFn = Encoding.utf8.checkEncodingFn,
         };
     }
 
@@ -368,6 +371,7 @@ pub fn StreamingDocument(comptime ReaderType: type) type {
             return .{
                 .context = doc,
                 .moveFn = &move,
+                .checkEncodingFn = Encoding.utf8.checkEncodingFn,
             };
         }
 
@@ -393,7 +397,7 @@ pub fn StreamingDocument(comptime ReaderType: type) type {
                 const new_buf_len = @min(min_buf_len, std.math.ceilPowerOfTwoAssert(usize, target_len));
                 doc.buf = try doc.gpa.realloc(doc.buf, new_buf_len);
             }
-            doc.avail += try doc.stream.read(doc.buf[doc.avail..]);
+            doc.avail += try doc.stream.readAll(doc.buf[doc.avail..]);
         }
     };
 }
@@ -428,6 +432,121 @@ test streamingDocument {
     try expectEqual(.eof, try reader.read());
 }
 
+pub fn EncodedDocument(comptime ReaderType: type) type {
+    return struct {
+        stream: ReaderType,
+        encoding: Encoding,
+        buf: []u8,
+        pos: usize,
+        avail: usize,
+        raw_buf: [4096]u8,
+        raw_buf_len: usize,
+        gpa: Allocator,
+
+        pub const Error = ReaderType.Error || Allocator.Error || error{InvalidEncoding};
+
+        pub fn init(gpa: Allocator, stream: ReaderType, encoding: Encoding) @This() {
+            return .{
+                .stream = stream,
+                .encoding = encoding,
+                .buf = &.{},
+                .pos = 0,
+                .avail = 0,
+                .raw_buf = undefined,
+                .raw_buf_len = 0,
+                .gpa = gpa,
+            };
+        }
+
+        pub fn deinit(doc: *@This()) void {
+            doc.gpa.free(doc.buf);
+            doc.* = undefined;
+        }
+
+        pub fn reader(doc: *@This(), gpa: Allocator, options: Reader.Options) GenericReader(Error) {
+            var modified_options = options;
+            modified_options.assume_valid_utf8 = true;
+            return .{ .reader = Reader.init(gpa, doc.source(), modified_options) };
+        }
+
+        pub fn source(doc: *@This()) Reader.Source {
+            return .{
+                .context = doc,
+                .moveFn = &move,
+                .checkEncodingFn = Encoding.utf8.checkEncodingFn,
+            };
+        }
+
+        fn move(context: *const anyopaque, advance: usize, len: usize) anyerror![]const u8 {
+            const doc: *@This() = @alignCast(@constCast(@ptrCast(context)));
+            doc.pos += advance;
+            if (len <= doc.avail - doc.pos) return doc.buf[doc.pos..][0..len];
+            doc.discardRead();
+            try doc.fillBuffer(len);
+            return doc.buf[0..@min(len, doc.avail)];
+        }
+
+        fn discardRead(doc: *@This()) void {
+            doc.avail -= doc.pos;
+            std.mem.copyForwards(u8, doc.buf[0..doc.avail], doc.buf[doc.pos..][0..doc.avail]);
+            doc.pos = 0;
+        }
+
+        const min_buf_len = 4096;
+
+        fn fillBuffer(doc: *@This(), target_len: usize) !void {
+            if (target_len > doc.buf.len) {
+                const new_buf_len = @min(min_buf_len, std.math.ceilPowerOfTwoAssert(usize, target_len));
+                doc.buf = try doc.gpa.realloc(doc.buf, new_buf_len);
+            }
+            while (doc.avail < target_len) {
+                doc.raw_buf_len += try doc.stream.readAll(doc.raw_buf[doc.raw_buf_len..]);
+                if (doc.raw_buf_len == 0) return;
+                const res = doc.encoding.transcode(doc.buf[doc.avail..], doc.raw_buf[0..doc.raw_buf_len]);
+                if (res.dest_written == 0) return error.InvalidEncoding;
+                doc.avail += res.dest_written;
+                std.mem.copyForwards(u8, doc.raw_buf[0..doc.raw_buf_len], doc.raw_buf[res.src_read..doc.raw_buf_len]);
+                doc.raw_buf_len -= res.src_read;
+            }
+        }
+
+        fn checkEncoding(context: *const anyopaque, encoding: []const u8) bool {
+            const doc: *const @This() = @alignCast(@ptrCast(context));
+            return doc.encoding.checkEncoding(encoding);
+        }
+    };
+}
+
+pub fn encodedDocument(gpa: Allocator, reader: anytype, encoding: Encoding) EncodedDocument(@TypeOf(reader)) {
+    return EncodedDocument(@TypeOf(reader)).init(gpa, reader, encoding);
+}
+
+test encodedDocument {
+    var fbs = std.io.fixedBufferStream(
+        \\<?xml version="1.0"?>
+        \\<root>Hello, world!</root>
+        \\
+    );
+    var doc = encodedDocument(std.testing.allocator, fbs.reader(), Encoding.utf8);
+    defer doc.deinit();
+    var reader = doc.reader(std.testing.allocator, .{});
+    defer reader.deinit();
+
+    try expectEqual(.xml_declaration, try reader.read());
+    try expectEqualStrings("1.0", reader.xmlDeclarationVersion());
+
+    try expectEqual(.element_start, try reader.read());
+    try expectEqualStrings("root", reader.elementName());
+
+    try expectEqual(.text, try reader.read());
+    try expectEqualStrings("Hello, world!", reader.textRaw());
+
+    try expectEqual(.element_end, try reader.read());
+    try expectEqualStrings("root", reader.elementName());
+
+    try expectEqual(.eof, try reader.read());
+}
+
 pub const Writer = @import("Writer.zig");
 
 pub fn GenericWriter(comptime SinkError: type) type {