aoc2022/3/1-2.zig

const std = @import("std");

// Wanted to implement both in a single modular and expansible way, so this is that.

const string = []const u8; // to simplify a bit
const Iterator = std.mem.SplitIterator(u8);
const stepFnType = *const fn (*Iterator, []string) ?[]string;

pub fn main() void {
    const input = @embedFile("input.asc");

    // The fundamental difference lies in the way of processing the file,
    // so we abstract that into a function to be able to pass its pointer.
    std.debug.print("Part 1: {}\n", .{sumPriorities(input, stepInline, 2)});
    std.debug.print("Part 2: {}\n", .{sumPriorities(input, stepInterline, 3)});
}

fn sumPriorities(input: string, stepFn: stepFnType, comptime amount: usize) usize {
    var totalPriorities: usize = 0;

    var iter = std.mem.split(u8, input, "\n");
    // This buffer is needed to not allocate but it shouldn't be used directly
    var _buffer = [_]string{undefined} ** amount;

    // Use the stepper to divide into slices properly
    while (stepFn(&iter, &_buffer)) |containers| {

        // If it wasn't found just contine with the next line
        const repe = findCommon(u8, containers) orelse continue;

        // Convert to priority as specified
        // 'a-z' -> '1-26', 'A-Z' -> '27-52'
        const priority = if (repe >= 'a') repe - 'a' + 1 else repe - 'A' + 27;

        totalPriorities += priority;
    }

    return totalPriorities;
}

// Helper to find the common needle in n haystacks
// Returns null if not found
fn findCommon(comptime T: type, haystacks: [][]const T) ?T {
    // Why
    if (haystacks.len <= 1) unreachable;

    // We need to find a common needle for all of the haystacks.
    // Compare each of the characters in the first one with
    // the other ones in sequence.
    //
    // - If a match was found, jump to the next haystack.
    // - If a match wasn't found, just to the next needle.

    needle: for (haystacks[0]) |needle| {
        // Semaphor to know if something was found when reaching
        // the end of loops.
        var found: bool = false;

        haystack: for (haystacks[1..]) |haystack| {
            found = false;

            for (haystack) |blade| {
                if (needle == blade) {
                    found = true;
                    break;
                }
            }

            if (!found) {
                // We didn't find the needle, try with another one
                continue :needle;
            } else {
                // We did find it!
                // Let's see if it's in the next haystack
                continue :haystack;
            }
        }

        // We found the needle in all haystacks, hooray!
        if (found) return needle;
    }

    // We ran out of needles and nothing was found
    return null;
}

// Steps the iterator and slices lines in the specified amount
fn stepInline(iter: *Iterator, buffer: []string) ?[]string {
    // We reached the end
    const line = iter.next() orelse return null;
    // Line isn't valid
    if (line.len == 0 or line.len % buffer.len != 0) return stepInline(iter, buffer);

    const len = line.len / buffer.len;
    for (buffer) |_, i| {
        // Split line into slices into buffer
        buffer[i] = line[i * len .. (i + 1) * len];
    }

    return buffer;
}

// Steps the iterator and puts whole lines in the specified amount
fn stepInterline(iter: *Iterator, buffer: []string) ?[]string {
    for (buffer) |_, i| {
        // No more lines
        const line = iter.next() orelse return null;
        // Invalid input, try next one
        if (line.len <= 0) return stepInterline(iter, buffer);

        buffer[i] = line;
    }

    return buffer;
}