From 3ad1c424d1762a769f8a063c0e212ec30a20a8a4 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <code@jdkaplan.dev>
Date: Fri, 6 Dec 2024 22:12:52 -0800
Subject: [PATCH] 2024 Day 7 Part 1

---
 2024/src/day07.zig | 136 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 136 insertions(+)
 create mode 100644 2024/src/day07.zig

diff --git a/2024/src/day07.zig b/2024/src/day07.zig
new file mode 100644
index 0000000..45ae225
--- /dev/null
+++ b/2024/src/day07.zig
@@ -0,0 +1,136 @@
+const std = @import("std");
+const aoc = @import("aoc.zig");
+
+const Allocator = std.mem.Allocator;
+const ArrayList = std.ArrayList;
+
+pub fn main() !void {
+    var gpa: std.heap.GeneralPurposeAllocator(.{}) = .{};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+
+    const file = try std.fs.cwd().openFile("input/day07.txt", .{});
+    defer file.close();
+
+    const text = try file.readToEndAlloc(allocator, comptime 1 << 30);
+    defer allocator.free(text);
+
+    var equations = try Equation.parseAll(allocator, text);
+    defer equations.deinit();
+    defer {
+        for (equations.items) |*eqn| eqn.deinit();
+    }
+
+    const stdout_file = std.io.getStdOut().writer();
+    var bw = std.io.bufferedWriter(stdout_file);
+    const stdout = bw.writer();
+
+    try stdout.print("{d}\n", .{try part1(allocator, equations.items)});
+    try bw.flush();
+}
+
+const Operator = enum {
+    Plus,
+    Times,
+
+    fn eval(self: Operator, a: u64, b: u64) u64 {
+        return switch (self) {
+            .Plus => a + b,
+            .Times => a * b,
+        };
+    }
+};
+
+const Operators = ArrayList(Operator);
+
+const Equation = struct {
+    value: u64,
+    operands: ArrayList(u64),
+
+    const Self = @This();
+
+    fn deinit(self: *Self) void {
+        self.operands.deinit();
+    }
+
+    fn parse(allocator: Allocator, line: []const u8) !Self {
+        const value, const rest = a: {
+            var it = std.mem.splitSequence(u8, line, ": ");
+            const value = try std.fmt.parseInt(u64, it.next().?, 10);
+            break :a .{ value, it.rest() };
+        };
+
+        const operands = a: {
+            var operands = std.ArrayList(u64).init(allocator);
+            var it = std.mem.tokenizeScalar(u8, rest, ' ');
+            while (it.next()) |num| {
+                const n = try std.fmt.parseInt(u64, num, 10);
+                try operands.append(n);
+            }
+            break :a operands;
+        };
+
+        return .{ .value = value, .operands = operands };
+    }
+
+    fn parseAll(allocator: Allocator, text: []const u8) !ArrayList(Self) {
+        var all = ArrayList(Self).init(allocator);
+
+        var it = std.mem.tokenizeScalar(u8, text, '\n');
+        while (it.next()) |line| {
+            const eqn = try Self.parse(allocator, line);
+            try all.append(eqn);
+        }
+
+        return all;
+    }
+
+    fn eval(self: Self, operators: ArrayList(Operator)) u64 {
+        var v = self.operands.items[0];
+        for (self.operands.items[1..], operators.items) |n, op| {
+            v = op.eval(v, n);
+        }
+        return v;
+    }
+
+    fn solvable(self: Self, allocator: Allocator) !bool {
+        var stack = ArrayList(Operators).init(allocator);
+        defer stack.deinit();
+        defer {
+            for (stack.items) |ops| ops.deinit();
+        }
+
+        try stack.append(Operators.init(allocator));
+
+        while (stack.popOrNull()) |ops| {
+            defer ops.deinit();
+
+            if (ops.items.len == self.operands.items.len - 1) {
+                if (self.eval(ops) == self.value) {
+                    return true;
+                }
+                continue;
+            }
+
+            var plus = try ops.clone();
+            try plus.append(Operator.Plus);
+            try stack.append(plus);
+
+            var times = try ops.clone();
+            try times.append(Operator.Times);
+            try stack.append(times);
+        }
+
+        return false;
+    }
+};
+
+fn part1(allocator: Allocator, equations: []Equation) !u64 {
+    var sum: u64 = 0;
+    for (equations) |eqn| {
+        if (try eqn.solvable(allocator)) {
+            sum += eqn.value;
+        }
+    }
+    return sum;
+}