adventofcode2025/10/code.js

import { readFileSync } from 'node:fs';

let sampleMode = false;
let usedArray = [];

const sampleArray = readFileSync('sample.txt').toString().split("\n");
const inputArray = readFileSync('input.txt').toString().split("\n");

if (sampleMode) {
  usedArray = sampleArray;
} else {
  usedArray = inputArray;
}

// Part One

console.time("part1");

let totalPresses = 0;

for (const element of usedArray) {
  const lights = element.match(/\[([.#]+)\]/)[1];
  const buttonMatches = [...element.matchAll(/\(([0-9,]+)\)/g)];
  const numLights = lights.length;

  // Target state
  const target = [...lights].map(c => c === '#' ? 1 : 0);

  // Parse buttons
  const buttons = buttonMatches.map(match => {
    const indices = match[1].split(',').map(Number);
    const effect = new Array(numLights).fill(0);
    for (const idx of indices) effect[idx] = 1;
    return effect;
  });

  const numButtons = buttons.length;
  let minPresses = Infinity;

  for (let mask = 0; mask < (1 << numButtons); mask++) {
    const state = new Array(numLights).fill(0);
    let presses = 0;

    for (let b = 0; b < numButtons; b++) {
      if (mask & (1 << b)) {
        presses++;
        for (let i = 0; i < numLights; i++) state[i] ^= buttons[b][i];
      }
    }

    if (state.every((v, i) => v === target[i]) && presses < minPresses) {
      minPresses = presses;
    }
  }

  totalPresses += minPresses;
}

console.timeEnd("part1");
console.log(totalPresses);


// Part Two

console.time("part2");

let totalPresses2 = 0;

for (const element of usedArray) {
  const buttonMatches = [...element.matchAll(/\(([0-9,]+)\)/g)];
  const joltageMatch = element.match(/\{([0-9,]+)\}/);

  const targets = joltageMatch[1].split(',').map(Number);
  const numCounters = targets.length;

  const buttons = buttonMatches.map(match => {
    const indices = match[1].split(',').map(Number);
    const effect = new Array(numCounters).fill(0);
    for (const idx of indices) {
      if (idx < numCounters) effect[idx] = 1;
    }
    return effect;
  });

  const numButtons = buttons.length;

  // Build augmented matrix [A | b] where A is transpose of buttons matrix
  // Rows = counters, Cols = buttons + 1 (for target)
  const matrix = [];
  for (let c = 0; c < numCounters; c++) {
    const row = [];
    for (let b = 0; b < numButtons; b++) {
      row.push(buttons[b][c]);
    }
    row.push(targets[c]);
    matrix.push(row);
  }

  // Gaussian elimination (over rationals, tracking which columns are pivots)
  const pivotCols = [];
  let pivotRow = 0;

  for (let col = 0; col < numButtons && pivotRow < numCounters; col++) {
    // Find pivot
    let maxRow = pivotRow;
    for (let row = pivotRow + 1; row < numCounters; row++) {
      if (Math.abs(matrix[row][col]) > Math.abs(matrix[maxRow][col])) {
        maxRow = row;
      }
    }

    if (matrix[maxRow][col] === 0) continue;

    // Swap rows
    [matrix[pivotRow], matrix[maxRow]] = [matrix[maxRow], matrix[pivotRow]];

    // Eliminate
    for (let row = 0; row < numCounters; row++) {
      if (row !== pivotRow && matrix[row][col] !== 0) {
        const factor = matrix[row][col] / matrix[pivotRow][col];
        for (let c = col; c <= numButtons; c++) {
          matrix[row][c] -= factor * matrix[pivotRow][c];
        }
      }
    }

    pivotCols.push(col);
    pivotRow++;
  }

  // Free variables are buttons not in pivotCols
  const freeVars = [];
  for (let b = 0; b < numButtons; b++) {
    if (!pivotCols.includes(b)) freeVars.push(b);
  }

  // Search over free variables with bounded range
  const maxFreeVal = Math.max(...targets);

  let bestPresses = Infinity;

  function searchFree(freeIdx, freeVals) {
    if (freeIdx === freeVars.length) {
      // Compute pivot variables from free variables
      const x = new Array(numButtons).fill(0);
      for (let i = 0; i < freeVars.length; i++) {
        x[freeVars[i]] = freeVals[i];
      }

      // Back-substitute to find pivot variables
      for (let i = pivotCols.length - 1; i >= 0; i--) {
        const col = pivotCols[i];
        let val = matrix[i][numButtons]; // RHS
        for (let c = col + 1; c < numButtons; c++) {
          val -= matrix[i][c] * x[c];
        }
        val /= matrix[i][col];
        x[col] = val;
      }

      // Check all non-negative integers
      let valid = true;
      let totalPresses = 0;
      for (let b = 0; b < numButtons; b++) {
        if (x[b] < -0.0001 || Math.abs(x[b] - Math.round(x[b])) > 0.0001) {
          valid = false;
          break;
        }
        x[b] = Math.round(x[b]);
        totalPresses += x[b];
      }

      if (valid && totalPresses < bestPresses) {
        // Verify solution
        let correct = true;
        for (let c = 0; c < numCounters; c++) {
          let sum = 0;
          for (let b = 0; b < numButtons; b++) {
            sum += buttons[b][c] * x[b];
          }
          if (sum !== targets[c]) {
            correct = false;
            break;
          }
        }
        if (correct) bestPresses = totalPresses;
      }
      return;
    }

    // Prune based on current sum
    const currentSum = freeVals.reduce((a, b) => a + b, 0);
    if (currentSum >= bestPresses) return;

    const maxVal = Math.min(maxFreeVal, bestPresses - currentSum);
    for (let v = 0; v <= maxVal; v++) {
      searchFree(freeIdx + 1, [...freeVals, v]);
    }
  }

  searchFree(0, []);
  totalPresses2 += bestPresses;
}

console.timeEnd("part2");
console.log(totalPresses2);

// functions