path: root/cpp/061channel.mu

# Mu synchronizes using channels rather than locks, like Erlang and Go.
#
# The two ends of a channel will usually belong to different routines, but
# each end should only be used by a single one. Don't try to read from or
# write to it from multiple routines at once.
#
# The key property of channels is that writing to a full channel or reading
# from an empty one will put the current routine in 'waiting' state until the
# operation can be completed.

scenario channel [
  run [
    1:address:channel <- init-channel 3:literal/capacity
    1:address:channel <- write 1:address:channel, 34:literal
    2:integer, 1:address:channel <- read 1:address:channel
  ]
  memory-should-contain [
    2 <- 34
  ]
]

container channel [
  # To avoid locking, writer and reader will never write to the same location.
  # So channels will include fields in pairs, one for the writer and one for the
  # reader.
  first-full:integer  # for write
  first-free:integer  # for read
  # A circular buffer contains values from index first-full up to (but not
  # including) index first-empty. The reader always modifies it at first-full,
  # while the writer always modifies it at first-empty.
  data:address:array:location
]

# result:address:channel <- init-channel capacity:integer
recipe init-channel [
  default-space:address:array:location <- new location:type, 30:literal
  # result = new channel
  result:address:channel <- new channel:type
  # result.first-full = 0
  full:address:integer <- get-address result:address:channel/deref, first-full:offset
  full:address:integer/deref <- copy 0:literal
  # result.first-free = 0
  free:address:integer <- get-address result:address:channel/deref, first-free:offset
  free:address:integer/deref <- copy 0:literal
  # result.data = new location[ingredient+1]
  capacity:integer <- next-ingredient
  capacity:integer <- add capacity:integer, 1:literal  # unused slot for 'full?' below
  dest:address:address:array:location <- get-address result:address:channel/deref, data:offset
  dest:address:address:array:location/deref <- new location:type, capacity:integer
  reply result:address:channel
]

# chan:address:channel <- write chan:address:channel, val:location
recipe write [
  default-space:address:array:location <- new location:type, 30:literal
  chan:address:channel <- next-ingredient
  val:location <- next-ingredient
  {
    # block if chan is full
    full:boolean <- channel-full? chan:address:channel
    break-unless full:boolean
    full-address:address:integer <- get-address chan:address:channel/deref, first-full:offset
    wait-for-location full-address:address:integer/deref
  }
  # store val
  circular-buffer:address:array:location <- get chan:address:channel/deref, data:offset
  free:address:integer <- get-address chan:address:channel/deref, first-free:offset
  dest:address:location <- index-address circular-buffer:address:array:location/deref, free:address:integer/deref
  dest:address:location/deref <- copy val:location
  # increment free
  free:address:integer/deref <- add free:address:integer/deref, 1:literal
  {
    # wrap free around to 0 if necessary
    len:integer <- length circular-buffer:address:array:location/deref
    at-end?:boolean <- greater-or-equal free:address:integer/deref, len:integer
    break-unless at-end?:boolean
    free:address:integer/deref <- copy 0:literal
  }
  reply chan:address:channel/same-as-ingredient:0
]

# result:location, chan:address:channel <- read chan:address:channel
recipe read [
  default-space:address:array:location <- new location:type, 30:literal
  chan:address:channel <- next-ingredient
  {
    # block if chan is empty
    empty:boolean <- channel-empty? chan:address:channel
    break-unless empty:boolean
    free-address:address:integer <- get-address chan:address:channel/deref, first-free:offset
    wait-for-location free-address:address:integer/deref
  }
  # read result
  full:address:integer <- get-address chan:address:channel/deref, first-full:offset
  circular-buffer:address:array:location <- get chan:address:channel/deref, data:offset
  result:location <- index circular-buffer:address:array:location/deref, full:address:integer/deref
  # increment full
  full:address:integer/deref <- add full:address:integer/deref, 1:literal
  {
    # wrap full around to 0 if necessary
    len:integer <- length circular-buffer:address:array:location/deref
    at-end?:boolean <- greater-or-equal full:address:integer/deref, len:integer
    break-unless at-end?:boolean
    full:address:integer/deref <- copy 0:literal
  }
  reply result:location, chan:address:channel/same-as-ingredient:0
]

scenario channel-initialization [
  run [
    1:address:channel <- init-channel 3:literal/capacity
    2:integer <- get 1:address:channel/deref, first-full:offset
    3:integer <- get 1:address:channel/deref, first-free:offset
  ]
  memory-should-contain [
    2 <- 0  # first-full
    3 <- 0  # first-free
  ]
]

scenario channel-write-increments-free [
  run [
    1:address:channel <- init-channel 3:literal/capacity
    1:address:channel <- write 1:address:channel, 34:literal
    2:integer <- get 1:address:channel/deref, first-full:offset
    3:integer <- get 1:address:channel/deref, first-free:offset
  ]
  memory-should-contain [
    2 <- 0  # first-full
    3 <- 1  # first-free
  ]
]

scenario channel-read-increments-full [
  run [
    1:address:channel <- init-channel 3:literal/capacity
    1:address:channel <- write 1:address:channel, 34:literal
    _, 1:address:channel <- read 1:address:channel
    2:integer <- get 1:address:channel/deref, first-full:offset
    3:integer <- get 1:address:channel/deref, first-free:offset
  ]
  memory-should-contain [
    2 <- 1  # first-full
    3 <- 1  # first-free
  ]
]

scenario channel-wrap [
  run [
    # channel with just 1 slot
    1:address:channel <- init-channel 1:literal/capacity
    # write and read a value
    1:address:channel <- write 1:address:channel, 34:literal
    _, 1:address:channel <- read 1:address:channel
    # first-free will now be 1
    2:integer <- get 1:address:channel/deref, first-free:offset
    3:integer <- get 1:address:channel/deref, first-free:offset
    # write second value, verify that first-free wraps
    1:address:channel <- write 1:address:channel, 34:literal
    4:integer <- get 1:address:channel/deref, first-free:offset
    # read second value, verify that first-full wraps
    _, 1:address:channel <- read 1:address:channel
    5:integer <- get 1:address:channel/deref, first-full:offset
  ]
  memory-should-contain [
    2 <- 1  # first-free after first write
    3 <- 1  # first-full after first read
    4 <- 0  # first-free after second write, wrapped
    5 <- 0  # first-full after second read, wrapped
  ]
]

## helpers

# An empty channel has first-empty and first-full both at the same value.
recipe channel-empty? [
  default-space:address:array:location <- new location:type, 30:literal
  chan:address:channel <- next-ingredient
  # return chan.first-full == chan.first-free
  full:integer <- get chan:address:channel/deref, first-full:offset
  free:integer <- get chan:address:channel/deref, first-free:offset
  result:boolean <- equal full:integer, free:integer
  reply result:boolean
]

# A full channel has first-empty just before first-full, wasting one slot.
# (Other alternatives: https://en.wikipedia.org/wiki/Circular_buffer#Full_.2F_Empty_Buffer_Distinction)
recipe channel-full? [
  default-space:address:array:location <- new location:type, 30:literal
  chan:address:channel <- next-ingredient
  # tmp = chan.first-free + 1
  tmp:integer <- get chan:address:channel/deref, first-free:offset
  tmp:integer <- add tmp:integer, 1:literal
  {
    # if tmp == chan.capacity, tmp = 0
    len:integer <- channel-capacity chan:address:channel
    at-end?:boolean <- greater-or-equal tmp:integer, len:integer
    break-unless at-end?:boolean
    tmp:integer <- copy 0:literal
  }
  # return chan.first-full == tmp
  full:integer <- get chan:address:channel/deref, first-full:offset
  result:boolean <- equal full:integer, tmp:integer
  reply result:boolean
]

# result:integer <- channel-capacity chan:address:channel
recipe channel-capacity [
  default-space:address:array:location <- new location:type, 30:literal
  chan:address:channel <- next-ingredient
  q:address:array:location <- get chan:address:channel/deref, data:offset
  result:integer <- length q:address:array:location/deref
  reply result:integer
]

scenario channel-new-empty-not-full [
  run [
    1:address:channel <- init-channel 3:literal/capacity
    2:integer <- channel-empty? 1:address:channel
    3:integer <- channel-full? 1:address:channel
  ]
  memory-should-contain [
    2 <- 1  # empty?
    3 <- 0  # full?
  ]
]

scenario channel-write-not-empty [
  run [
    1:address:channel <- init-channel 3:literal/capacity
    1:address:channel <- write 1:address:channel, 34:literal
    2:integer <- channel-empty? 1:address:channel
    3:integer <- channel-full? 1:address:channel
  ]
  memory-should-contain [
    2 <- 0  # empty?
    3 <- 0  # full?
  ]
]

scenario channel-write-full [
  run [
    1:address:channel <- init-channel 1:literal/capacity
    1:address:channel <- write 1:address:channel, 34:literal
    2:integer <- channel-empty? 1:address:channel
    3:integer <- channel-full? 1:address:channel
  ]
  memory-should-contain [
    2 <- 0  # empty?
    3 <- 1  # full?
  ]
]

scenario channel-read-not-full [
  run [
    1:address:channel <- init-channel 1:literal/capacity
    1:address:channel <- write 1:address:channel, 34:literal
    _, 1:address:channel <- read 1:address:channel
    2:integer <- channel-empty? 1:address:channel
    3:integer <- channel-full? 1:address:channel
  ]
  memory-should-contain [
    2 <- 1  # empty?
    3 <- 0  # full?
  ]
]