// ============================================================================
// ROT13 — self-inverse letter rotation
// ============================================================================
//
// Rotates each letter by 13, preserving case; applying it twice restores the
// original. Demonstrates two alphabet lookups (lower/upper), index_of for the
// source position, and the involution property in a test.
//
// Run: soma run rot13.cell --signal rot13 "Hello, World"

cell Rot13 {
    face {
        signal rot13(text: String) -> String
    }

    on rot13(text: String) {
        let lower = "abcdefghijklmnopqrstuvwxyz"
        let upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        let out = ""
        for i in range(0, len(text)) {
            let c = text[i]
            let lp = index_of(lower, c)
            let up = index_of(upper, c)
            if lp >= 0 {
                out = out + lower[(lp + 13) % 26]
            } else if up >= 0 {
                out = out + upper[(up + 13) % 26]
            } else {
                out = out + c
            }
        }
        return out
    }
}

cell test Rot13Test {
    rules {
        assert rot13("hello") == "uryyb"
        assert rot13("Hello, World") == "Uryyb, Jbeyq"
        assert rot13(rot13("Self inverse 123!")) == "Self inverse 123!"
        assert rot13("ABC") == "NOP"
        assert rot13("123") == "123"
    }
}