🧩 Skyscraper Puzzle Solver (4×4)

This project implements a 4×4 Skyscraper puzzle solver in pure C.
It follows the same logic used in the 42 Piscine project rush01 — using backtracking, visibility checking, and constraint validation.

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🏙️ 1. What Is the Skyscraper Puzzle?

Imagine a city grid of 4 × 4 buildings.
Each cell must contain a building height from 1 to 4, and every row/column must include all four heights (no duplicates).

Around the grid, there are 16 clues that tell you how many buildings you can “see” from that side —
taller buildings hide the shorter ones behind them.

If you stand on the left side of a row and can see 3 buildings,
that means the sequence (from left → right) increases in height three times.

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⚙️ 2. Input Format (The 16 Clues)

The program expects one argument containing 16 numbers (1–4) separated by spaces:

Internally, the clues are stored as:

Row	Meaning	Example
0	Top clues	4 3 2 1
1	Bottom clues	1 2 2 2
2	Left clues	4 3 2 1
3	Right clues	1 2 2 2

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🧠 3. Program Flow Overview

The solver works in three main stages:

1. Parser (parser.c)
   – Reads and validates 16 clues from the command line.
2. Solver (solver.c)
   – Recursively fills the 4×4 grid, one cell at a time.
3. Constraint Checkers
   • is_unique() → no duplicates in row/col
   • is_safe() → combines uniqueness + partial visibility
   • check_visibility() → validates final solution
4. Display (main.c)
   – Prints the solved grid or "Error" if impossible.

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🔁 4. Backtracking Algorithm Explained

Backtracking = “try, test, undo, retry.”

Every call represents a cell.
If the solver finds a contradiction, it undoes the move and tries the next number.

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👁️ 5. Visibility Rule (count_visible)

From any side, count how many buildings are visible.

Example line: [2 1 4 3]

• Left → Right view:
  2 (see 1) (see 4 since 4 > 2) (ignore 3 since 3 < 4)
  → 3 visible
• Right → Left view: [3 4 1 2] → 2 visible

This rule ensures each row/column satisfies the side clues.

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🧩 6. File-by-File Breakdown

File	Purpose
main.c	Handles input, initializes grid, calls solver, prints result
parser.c	Converts 16-clue string into 4×4 array
unique_check.c	Ensures no duplicates in rows/columns
visibility_check.c	Counts visible buildings & verifies final grid
fill_check.c	Combines uniqueness + partial visibility for safety checks
solver.c	Backtracking recursion engine

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🔧 7. Build & Run

Compile manually

cc -Wall -Wextra -Werror *.c -o rush01

             ┌────────────────────┐
             │   main()           │
             │ parse_clues(argv)  │
             │ init grid[4][4]    │
             └────────┬───────────┘
                      ↓
               ┌─────────────┐
               │  solve()    │
               │ recursion   │
               └─────┬───────┘
                     ↓
            ┌─────────────────┐
            │  is_safe()      │
            │  - is_unique()  │
            │  - check_line() │
            └─────────────────┘
                     ↓
             if success → print grid
             else → "Error"