Data

Evaluate an expression with cache

The algorithmic skeleton is as follows:

if don't cache result then
  return eval(expr)
if this.current.contains(config, expr) then
  return cached value
else
  val assumed = this.last(config, expr) or bottom value if absent
  this.current(config, expr) = assumed
  val actual = eval(expr)

  if assumed != actual then
    this.changed = true
    this.current(config, expr) = actual

Prepare cache for the next iteration

1. Reset changed flag.

2. Use current cache as last cache and set current cache to be empty.