Many changes to timer.

[abersnaze]

Started out as a small change to allow reentrant timer scheduling but ended up rewriting most of it.

• changed to indirectly indexing tasks by min heap to make tick O(1) and scheduling O(log(n))
• now supported reentrant scheduling of tasks in handlers.
• fix a bug in cancel. if a task completes and slot reused.
• added a debugging mode ifdef.

[abersnaze]

Here is some code I was using to test it.

#include "Timer.h"

auto timer = timer_create_default();

bool off(void*, long) {
  digitalWrite(LED_BUILTIN, LOW);
}

bool on(void*, long overdue_by) {
  digitalWrite(LED_BUILTIN, HIGH);
  timer.in(random(250 - overdue_by, 750 - overdue_by), off);
  return true;
}

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, LOW);
  timer.every(1000, on);
}

void loop() {
  timer.tick();
}

[contrem]

Nice work. However, I think min heap is overkill for the following reasons:

• The increase in complexity for min heap limits the number of people that can understand the code.
• The number of timers is too small to merit a heap. It currently defaults to 16. The overhead of maintaining the heap likely outweighs any benefits.
• The number of timers is a compile time constant, so the current implementation is technically already constant time, just a slightly larger constant.

I'm not sure that your checks handle timer overflow. Were you able to test against it?

If possible, could you provide test cases for the re-entrant and cancel problems you experienced?

Thanks for your time.