# Serial echo example
# Mirrors Arduino echo.ino and CircuitPython echo.py
#
# Run with Thonny (set interpreter to MicroPython (Raspberry Pi Pico)).
# Type a line in the Thonny Shell and press Enter — the board reads the
# line and sends it straight back, prefixed with "echo: ".
#
# Key concepts
# ------------
#   sys.stdin / sys.stdout  — standard input/output streams, connected to the
#                             USB serial port on the XIAO RP2040
#   select.select()         — checks whether a stream has data ready to read
#                             WITHOUT blocking; essential for keeping the main
#                             loop responsive while waiting for input
#   sys.stdin.readline()    — reads bytes until '\n' and returns them as a str;
#                             safe to call after select confirms data is waiting

import sys
import select

print("XIAO RP2040 ready — type something and press Enter.")

while True:
    # select.select(rlist, wlist, xlist, timeout)
    #   rlist   — streams to watch for readable data   → [sys.stdin]
    #   wlist   — streams to watch for writable space  → [] (unused)
    #   xlist   — streams to watch for errors          → [] (unused)
    #   timeout — seconds to wait; 0 = return immediately (non-blocking)
    #
    # Returns three lists matching the inputs.  If readable is non-empty,
    # at least one byte is waiting in sys.stdin.
    readable, _, _ = select.select([sys.stdin], [], [], 0)

    if readable:
        # readline() reads up to and including the next '\n'.
        # Because select confirmed data is available, this will not block.
        line = sys.stdin.readline()

        # sys.stdout.write() sends text without appending an extra newline,
        # so the '\n' already inside 'line' terminates the echoed line.
        sys.stdout.write("echo: " + line)