Fold a string into a triangle

triangle.cc

// triangle -- Fold a string into a triangle.
//
// From https://codegolf.stackexchange.com/questions/127649/fold-a-string-into-a-triangle
//
// By Jaldhar H. Vyas <jaldhar@braincells.com>
// Copyright (C) 2017, Consolidated Braincells Inc.  All rights reserved.
// "Do what thou wilt shall be the whole of the license."
//
// Given a string whose length is divisible by 4, make a triangle as
// demonstrated below.
//
// If the string is abcdefghijkl, then the triangle would be:
//
//       a
//      b l
//     c   k
//    defghij
//
// If the string is iamastringwithalengthdivisblebyfour, then the triangle would
// be:
//              i
//             a r
//            m   u
//           a     o
//          s       f
//         t         y
//        r           b
//       i             e
//      n               l
//     gwithalengthdivisib
//
// If the string is thisrepresentationisnotatriangle, then the triangle would
// be:
//             t
//            h e
//           i   l
//          s     g
//         r       n
//        e         a
//       p           i
//      r             r
//     esentationisnotat
//
// To compile: c++ -std=c++14 -O2 -g -Wall -o triangle triangle.cc
//

#include <cstdlib>
#include <iostream>
#include <string>
using namespace std;

int main(int argc, const char* argv[]) {
    if (argc < 2) {
        cerr << "Usage: triangle \"string\"" << endl;
        return EXIT_FAILURE;
    }

    string input(argv[1]);
    size_t len = input.length();

    if (len % 4 != 0) {
        cerr << "Length of string must be divisble by 4" << endl;
        return EXIT_FAILURE;
    }

    size_t n = len / 4;
    size_t left = 1;
    size_t right = len - 1;

    // Top of the triangle
    cout.width(n + 1);
    cout << input[0] << '\n';

    // Intermediate lines
    for (size_t i = 1, offset = n, gap = 2; i < n; i++, offset--, gap += 2) {
        cout.width(offset);
        cout << input[left++];
        cout.width(gap);
        cout << input[right--] << '\n';
    }

    // Base of the triangle
    for (size_t i = left; i <= right; i++) {
        cout << input[i];
    }
    cout << endl;

    return EXIT_SUCCESS;
}