nextRecord

fn nextRecord(bc: *BitcodeReader) !?Record {
    const state = &bc.stack.items[bc.stack.items.len - 1];
    const abbrev_id = bc.readFixed(u32, state.abbrev_id_width) catch |err| switch (err) {
        error.EndOfStream => return null,
        else => |e| return e,
    };
    if (abbrev_id >= state.abbrevs.abbrevs.items.len) return error.InvalidAbbrevId;
    const abbrev = state.abbrevs.abbrevs.items[abbrev_id];

    var record_arena = bc.record_arena.promote(bc.allocator);
    defer bc.record_arena = record_arena.state;
    _ = record_arena.reset(.retain_capacity);

    var operands = try std.array_list.Managed(u64).initCapacity(record_arena.allocator(), abbrev.operands.len);
    var blob = std.array_list.Managed(u8).init(record_arena.allocator());
    for (abbrev.operands, 0..) |abbrev_operand, abbrev_operand_i| switch (abbrev_operand) {
        .literal => |value| operands.appendAssumeCapacity(value),
        .encoding => |abbrev_encoding| switch (abbrev_encoding) {
            .fixed => |width| operands.appendAssumeCapacity(try bc.readFixed(u64, width)),
            .vbr => |width| operands.appendAssumeCapacity(try bc.readVbr(u64, width)),
            .array => |len_width| {
                assert(abbrev_operand_i + 2 == abbrev.operands.len);
                const len: usize = @intCast(try bc.readVbr(u32, len_width));
                try operands.ensureUnusedCapacity(len);
                for (0..len) |_| switch (abbrev.operands[abbrev.operands.len - 1]) {
                    .literal => |elem_value| operands.appendAssumeCapacity(elem_value),
                    .encoding => |elem_encoding| switch (elem_encoding) {
                        .fixed => |elem_width| operands.appendAssumeCapacity(try bc.readFixed(u64, elem_width)),
                        .vbr => |elem_width| operands.appendAssumeCapacity(try bc.readVbr(u64, elem_width)),
                        .array, .blob => return error.InvalidArrayElement,
                        .char6 => operands.appendAssumeCapacity(try bc.readChar6()),
                    },
                    .align_32_bits, .block_len => return error.UnsupportedArrayElement,
                    .abbrev_op => switch (try bc.readFixed(u1, 1)) {
                        1 => try operands.appendSlice(&.{
                            Abbrev.Operand.literal_id,
                            try bc.readVbr(u64, 8),
                        }),
                        0 => {
                            const encoding: Abbrev.Operand.Encoding =
                                @fromBackingInt(@intCast(try bc.readFixed(u3, 3)));
                            try operands.append(@backingInt(encoding));
                            switch (encoding) {
                                .fixed, .vbr => try operands.append(try bc.readVbr(u7, 5)),
                                .array, .char6, .blob => {},
                                _ => return error.UnsuportedAbbrevEncoding,
                            }
                        },
                    },
                };
                break;
            },
            .char6 => operands.appendAssumeCapacity(try bc.readChar6()),
            .blob => |len_width| {
                assert(abbrev_operand_i + 1 == abbrev.operands.len);
                const len = std.math.cast(usize, try bc.readVbr(u32, len_width)) orelse
                    return error.Overflow;
                bc.align32Bits();
                try bc.readBytes(try blob.addManyAsSlice(len));
                bc.align32Bits();
            },
        },
        .align_32_bits => bc.align32Bits(),
        .block_len => operands.appendAssumeCapacity(try bc.read32Bits()),
        .abbrev_op => unreachable,
    };
    return .{
        .name = name: {
            if (operands.items.len < 1) break :name &.{};
            const record_id = std.math.cast(u32, operands.items[0]) orelse break :name &.{};
            if (state.block_id) |block_id| {
                if (bc.block_info.get(block_id)) |block_info| {
                    break :name block_info.record_names.get(record_id) orelse break :name &.{};
                }
            }
            break :name &.{};
        },
        .id = std.math.cast(u32, operands.items[0]) orelse return error.InvalidRecordId,
        .operands = operands.items[1..],
        .blob = blob.items,
    };
}