A program that makes far more than it keeps

# memory.zeno: a program that makes far more than it keeps.
#
#     zeno run examples/memory.zeno              run it with the interpreter
#     zeno build --memory examples/memory.zeno   compile it, with a memory report
#     ./memory
#
# A logger takes a reading every tick. Each reading is an object with some
# text in it, so every tick takes a little memory. Only the last few
# readings and the hottest one are kept. The rest is garbage, and the
# garbage collector in the compiled program gives that memory back.

class Reading {
    tick: int
    celsius: float
    note: str
}

function describe(reading: Reading) -> str {
    return f"tick {reading.tick}: {reading.celsius:.1f} C ({reading.note})"
}

var recent: list[Reading] = []          # the last few readings
var hottest: Reading or none = none
var lines = 0

for tick in range(50000) {
    let celsius = 20.0 + (tick * 37 % 151) / 10.0
    let reading = Reading(tick, celsius, f"sample {tick}")

    # Start a new list every five readings. The old list, and the readings
    # only it knew about, can no longer be reached from any variable.
    if len(recent) == 5 {
        recent = []
    }
    recent.append(reading)

    if hottest is none or reading.celsius > hottest.celsius {
        hottest = reading
    }

    # Text that is made, measured and forgotten at once.
    lines += len(describe(reading).split())
}

print("words logged:", lines)
print("still held:")
for reading in recent {
    print("  " + describe(reading))
}
if hottest is not none {
    print("hottest: " + describe(hottest))
}

# Built with --memory, the program ends with a line like this one:
#
#   memory: 780027 blocks made (13222784 bytes), at most 1048576 bytes in use at once, 12 collections
#
# Thirteen megabytes were asked for. One megabyte at a time was enough.

words logged: 300000
still held:
  tick 49995: 26.5 C (sample 49995)
  tick 49996: 30.2 C (sample 49996)
  tick 49997: 33.9 C (sample 49997)
  tick 49998: 22.5 C (sample 49998)
  tick 49999: 26.2 C (sample 49999)
hottest: tick 102: 35.0 C (sample 102)