Solve day 3

src/day03.zig

@@ -0,0 +1,145 @@
+const std = @import("std");
+const util = @import("util.zig");
+
+pub fn main() !void {
+    const input = @embedFile("data/day03.txt");
+    const sln = try solve(util.gpa, input);
+    std.debug.print("{d}\n", .{sln.sum});
+    std.debug.print("{d}\n", .{sln.gears});
+}
+
+const Part = struct {
+    value: usize,
+    line: usize,
+    start: usize,
+    end: usize,
+};
+
+const Symbol = struct { char: u8, line: usize, offset: usize };
+
+const Solution = struct {
+    sum: usize,
+    gears: usize,
+};
+
+fn absDiff(comptime T: type, a: T, b: T) T {
+    return @max(a, b) - @min(a, b);
+}
+
+fn solve(alloc: std.mem.Allocator, input: []const u8) !Solution {
+    var parts = std.ArrayList(Part).init(alloc);
+    var symbols = std.ArrayList(Symbol).init(alloc);
+
+    defer parts.deinit();
+    defer symbols.deinit();
+
+    // Accumulate parts and symbols.
+    var it = std.mem.splitScalar(u8, input, '\n');
+    var lnum: usize = 0;
+    while (it.next()) |line| : (lnum += 1) {
+        var current: Part = undefined;
+        var in_part = false;
+
+        var offset: usize = 0;
+        for (line) |char| {
+            switch (char) {
+                '0'...'9' => {
+                    if (!in_part) {
+                        current.line = lnum;
+                        current.start = offset;
+                        current.value = 0;
+                        in_part = true;
+                    }
+                    current.value = current.value * 10 + (char - '0');
+                },
+                '.' => {
+                    if (in_part) {
+                        current.end = offset - 1;
+                        try parts.append(current);
+                    }
+                    in_part = false;
+                },
+                else => {
+                    if (in_part) {
+                        current.end = offset - 1;
+                        try parts.append(current);
+                    }
+                    in_part = false;
+                    try symbols.append(.{ .char = char, .line = lnum, .offset = offset });
+                },
+            }
+            offset += 1;
+        }
+
+        // Handle number at the end of a line.
+        if (in_part) {
+            current.end = offset - 1;
+            try parts.append(current);
+        }
+    }
+
+    // Locate actual parts.
+    var sum: usize = 0;
+    var gear_sum: usize = 0;
+    var gears = std.AutoHashMap(Symbol, Part).init(alloc);
+    defer gears.deinit();
+
+    ploop: for (parts.items) |part| {
+        // std.debug.print("part {d}x{d}..{d}\n", .{ part.line, part.start, part.end });
+        for (symbols.items) |symbol| {
+            // std.debug.print("symbol {d}x{d}\n", .{ symbol.line, symbol.offset });
+            const inOffset = part.start <= symbol.offset + 1 and part.end + 1 >= symbol.offset;
+            const inLine = absDiff(usize, part.line, symbol.line) <= 1;
+            if (inOffset and inLine) {
+                // std.debug.print("part {d}x({d}..{d})={d}\n", .{ part.line, part.start, part.end, part.value });
+                sum += part.value;
+
+                // Detect gears.
+                if (symbol.char == '*') {
+                    if (gears.get(symbol)) |other_gear| {
+                        gear_sum += part.value * other_gear.value;
+                        _ = gears.remove(symbol);
+                    } else {
+                        try gears.put(symbol, part);
+                    }
+                }
+
+                continue :ploop;
+            }
+        }
+    }
+
+    return .{ .sum = sum, .gears = gear_sum };
+}
+
+test "silver" {
+    const input =
+        \\467..114..
+        \\...*......
+        \\..35..633.
+        \\......#...
+        \\617*......
+        \\.....+.58.
+        \\..592.....
+        \\......755.
+        \\...$.*....
+        \\664..598..
+    ;
+    try std.testing.expectEqual(@as(usize, 4361), (try solve(std.testing.allocator, input)).sum);
+}
+
+test "gold" {
+    const input =
+        \\467..114..
+        \\...*......
+        \\..35..633.
+        \\......#...
+        \\617*......
+        \\.....+.58.
+        \\..592.....
+        \\......755.
+        \\...$.*....
+        \\.664.598..
+    ;
+    try std.testing.expectEqual(@as(usize, 467835), (try solve(std.testing.allocator, input)).gears);
+}