Read slice of T from AnyReader

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1

I’m trying to read a slice of T from a buffer of bytes without copying or allocating.
The length of the slice is encoded in the stream itself and not known at comptime.

As far as I can tell this is not possible with the reader interface in the std, but I can’t implement it myself because I can’t figure out how to read the current position of the reader context.

const std = @import("std");
const builtin = @import("builtin");

const content: [4]u32 = .{ 0x3, 0xfa, 0xfb, 0xfc };
const stream: *const [4 * 4]u8 = @ptrCast(&content);

pub fn main() !void {
    var fst = std.io.fixedBufferStream(stream);
    const vec = try readVector(u32, fst.reader().any(), stream);
    std.debug.print("{any}\n", .{vec});
}

fn readVector(T: type, rd: std.io.AnyReader, buf: []const u8) ![]const T {
    const length = try rd.readInt(u32, .little);
    const offset = rd.context.i; // can't use the underlying context because it's *anyopaque

    const p: [*]const T = @alignCast(@ptrCast(buf.ptr + offset));
    return p[0..length];
}
2

After thinking about it, it makes more sense that AnyReader doesn’t expose the current position. Maybe FixedBufferStream is more fit for the task

3

This is work around.
You can use std.io.StreamSource type to resolve this issue.

const std = @import("std");
const builtin = @import("builtin");

const content: [4]u32 = .{ 0x3, 0xfa, 0xfb, 0xfc };
const stream: *const [4 * 4]u8 = @ptrCast(&content);

pub fn main() !void {
    // Wrap `std.io.fixedBufferStream` with `std.io.StreamSource`
    var source = std.io.StreamSource{ .const_buffer = std.io.fixedBufferStream(stream) };
    // Instead of `AnyReader`, It passes `std.io.StreamSource` instance
    const vec = try readVector(u32, &source, stream);
    std.debug.print("{any}\n", .{vec});
}

// I've changed `T` to `comptime T`
fn readVector(comptime T: type, source: *std.io.StreamSource, buf: []const u8) ![]const T {
    const rd = source.reader().any();
    const length = try rd.readInt(T, .little);
    
    // You can access to offset via `SeekableStream` in `std.io.StreamSource`
    const offset = try source.getPos();

    const p: [*]const T = @alignCast(@ptrCast(buf.ptr + offset));
    return p[0..length];
}
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