Solved day 7. A bit of recursion!

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

src/day07.zig

@@ -0,0 +1,73 @@
+const std = @import("std");
+const print = std.debug.print;
+const fmt = std.fmt;
+const args = @import("lib/args.zig");
+const files = @import("lib/files.zig");
+
+fn isEquationValid(operands: []u64, acc: u64, target: u64) bool {
+    if (operands.len == 0) {
+        return acc == target;
+    }
+    if (acc > target) {
+        return false;
+    }
+    return isEquationValid(operands[1..], acc * operands[0], target) or
+        isEquationValid(operands[1..], acc + operands[0], target);
+}
+
+fn concatIntegers(a: u64, b: u64) u64 {
+    return (a * std.math.pow(u64, 10, std.math.log10(b) + 1)) + b;
+}
+
+fn isEquationValidPart2(operands: []u64, acc: u64, target: u64) bool {
+    if (operands.len == 0) {
+        return acc == target;
+    }
+    if (acc > target) {
+        return false;
+    }
+    return isEquationValidPart2(operands[1..], acc * operands[0], target) or
+        isEquationValidPart2(operands[1..], concatIntegers(acc, operands[0]), target) or
+        isEquationValidPart2(operands[1..], acc + operands[0], target);
+}
+
+// equationValue takes an equation in string
+// form and returns its calibration value
+// if it can be made valid, otherwise, it
+// returns null.
+fn equationValue(buf: []u8) !?u64 {
+    var target: u64 = undefined;
+    var operands_buf: [16]u64 = undefined;
+    var operands_count: usize = 0;
+
+    var ptr: usize = 0;
+    while (buf[ptr] != ':') : (ptr += 1) {}
+    target = try fmt.parseUnsigned(u64, buf[0..ptr], 10);
+
+    // Skip colon and space.
+    ptr += 2;
+    while (ptr < buf.len) {
+        defer operands_count += 1;
+        const start = ptr;
+        while (ptr < buf.len and buf[ptr] != ' ') : (ptr += 1) {}
+        operands_buf[operands_count] = try fmt.parseUnsigned(u64, buf[start..ptr], 10);
+        ptr += 1; // Skip the space.
+    }
+    return if (isEquationValidPart2(operands_buf[0..operands_count], 0, target)) target else 0;
+}
+
+pub fn main() !void {
+    const input_path = args.getFirstArg();
+    const input = try files.openForReading(input_path);
+    defer input.close();
+
+    var calibration: u64 = 0;
+    var line_buf: [64]u8 = undefined;
+    while (files.readLine(input, &line_buf)) |line| {
+        calibration += (try equationValue(line)) orelse 0;
+    } else |err| {
+        print("done reading input file with error: {any}\n", .{err});
+    }
+
+    print("Calibration value: {d}\n", .{calibration});
+}