2024 day 1

2024/01/solve.zig

@@ -0,0 +1,74 @@
+const std = @import("std");
+const utils = @import("utils.zig");
+
+const NumberList = std.ArrayList(u32);
+const Histogram = std.AutoHashMap(u32, u32);
+
+const Day01 = struct {
+    left: NumberList,
+    right: NumberList,
+
+    pub fn release(self: @This()) void {
+        self.left.deinit();
+        self.right.deinit();
+    }
+};
+
+fn parse(allocator: std.mem.Allocator, input: anytype) !Day01 {
+    var left = try NumberList.initCapacity(allocator, 1000);
+    var right = try NumberList.initCapacity(allocator, 1000);
+    var lines = utils.lineIterator(input);
+
+    while (lines.next()) |line| {
+        var columns = std.mem.tokenizeScalar(u8, line, ' ');
+        try left.append(try std.fmt.parseInt(u32, columns.next().?, 10));
+        try right.append(try std.fmt.parseInt(u32, columns.next().?, 10));
+    }
+
+    return .{ .left = left, .right = right };
+}
+
+fn part1(input: Day01) u64 {
+    var result: u64 = 0;
+
+    std.sort.pdq(u32, input.left.items, {}, std.sort.asc(u32));
+    std.sort.pdq(u32, input.right.items, {}, std.sort.asc(u32));
+
+    for (input.left.items, input.right.items) |a, b| {
+        result += if (a > b) (a - b) else (b - a);
+    }
+    return result;
+}
+
+fn part2(allocator: std.mem.Allocator, input: Day01) !u64 {
+    var count = Histogram.init(allocator);
+    defer count.deinit();
+
+    var result: u64 = 0;
+
+    for (input.right.items) |b| {
+        try count.put(b, 1 + (count.get(b) orelse 0));
+    }
+
+    for (input.left.items) |a| {
+        result += a * (count.get(a) orelse 0);
+    }
+
+    return result;
+}
+
+pub fn main() !void {
+    var gpa: std.heap.GeneralPurposeAllocator(.{}) = .{};
+    // this line is what logs memory leaks on exit
+    defer _ = gpa.deinit();
+    // this allocator is how we actually allocate memory
+    const allocator = gpa.allocator();
+
+    var stdin = std.io.bufferedReader(std.io.getStdIn().reader());
+
+    const input = try parse(allocator, stdin.reader());
+    defer input.release();
+
+    std.debug.print("lists total distance  : {:10}\n", .{part1(input)});
+    std.debug.print("lists similarity score: {:10}\n", .{try part2(allocator, input)});
+}