# 01: Hello world on the VDG text screen.
#
# Exercises: memory-mapped globals, fixed arrays, `for`/`in`,
# integer literals, hex literals, the divergent main loop.

# The CoCo 2 text screen: 32x16 cells, one byte per cell, at $0400.
# `at` pins a global to an absolute address; the compiler allocates
# nothing for it. This is a language feature, not a library, on this
# target.
var screen: [Uint8; 512] at 0x0400

const COLS: Uint16 = 32

fn put_char(row: Uint16, col: Uint16, ch: Uint8) {
    # The compiler can't prove `row * COLS + col < 512` here, so this
    # index gets a runtime bounds check. That's the escape hatch, not
    # the idiom -- see `main` for the checked-once-per-loop style.
    screen[row * COLS + col] = ch
}

fn main() {
    # VDG character codes, not ASCII. A real program would use a
    # conversion table; string literal semantics are an open question.
    let msg: [Uint8; 5] = [0x08, 0x05, 0x0C, 0x0C, 0x0F]  # "HELLO"

    # `for x in array` iterates by value; the compiler proves bounds
    # once for the whole loop. No index, no check.
    var col: Uint16 = 0
    for ch in msg {
        put_char(0, col, ch + 0x40)
        col = col + 1
    }

    # `loop` is an infinite loop; it types as Never, so `main` is
    # allowed to not return.
    loop {}
}