aoc2025/day3/day3b.zig

const std = @import("std");

var allocatorBuffer: [10*1024*1024]u8 = undefined;

fn findLargestDigitPos(batteryLine: []const u8) usize {
    var largestPos: usize = 0;
    var largestDigit: u8 = 0;

    for (batteryLine, 0..) |digit, pos| {
        if (digit > largestDigit) {
            largestDigit = digit;
            largestPos = pos;
        }
    }

    return largestPos;
}

pub fn main() !void {
    var fb_alloc = std.heap.FixedBufferAllocator.init(&allocatorBuffer);
    var allocator = fb_alloc.allocator();

    var printBuf: [1024]u8 = undefined;

    const inputFile = try std.fs.cwd().openFile(
        "input.txt",
        .{
            .mode = .read_only,
        },
    );
    defer inputFile.close();

    var inputFileBuffer: [1024]u8 = undefined;
    var inputFileReader = inputFile.reader(&inputFileBuffer);
    const inputFileSize = try inputFileReader.getSize();

    const inputBuffer = try allocator.alloc(u8, inputFileSize);
    try inputFileReader.interface.readSliceAll(inputBuffer);

    var counter:u64 = 0;

    var it = std.mem.tokenizeScalar(u8, inputBuffer, '\n');
    while (it.next()) |batteryLine| {
        var joltage:u64 = 0;
        var remainderLine = batteryLine;
        for (0..12) |digitOffset| {
            const minSuffix:usize = @intCast(12 - digitOffset - 1);
            const largestPos = findLargestDigitPos(remainderLine[0..remainderLine.len-minSuffix]);
            joltage *= 10;
            joltage +=  remainderLine[largestPos] - '0';
            remainderLine = remainderLine[largestPos+1..];
        }
        counter += joltage;
    }

    try std.fs.File.stdout().writeAll(
      try std.fmt.bufPrint(&printBuf, "{}\n", .{counter}));
}