Lua min heap

heap.lua

---
-- Lua min heap
-- Based on http://lua-users.org/lists/lua-l/2007-07/msg00482.html
-- @author Luis Carvalho <carvalho@dam.brown.edu>

local assert, setmetatable = assert, setmetatable

local M = {}

local function push (h, k, v)
  assert(v ~= nil, "cannot push nil")
  local t = h.nodes
  local h = h.heap
  local n = #h + 1 -- node position in heap array (leaf)
  local p = (n - n % 2) / 2 -- parent position in heap array
  h[n] = k -- insert at a leaf
  t[k] = v
  while n > 1 and t[h[p]] > v do -- climb heap?
    h[p], h[n] = h[n], h[p]
    n = p
    p = (n - n % 2) / 2
  end
end

local function peek (h)
  local t = h.nodes
  local h = h.heap
  local s = #h
  assert(s > 0, "cannot pop from empty heap")
  local e = h[1] -- min (heap root)
  local r = t[e]
  return e, r
end

local function pop (h)
  local t = h.nodes
  local h = h.heap
  local s = #h
  assert(s > 0, "cannot pop from empty heap")
  local e = h[1] -- min (heap root)
  local r = t[e]
  local v = t[h[s]]
  h[1] = h[s] -- move leaf to root
  h[s] = nil -- remove leaf
  t[e] = nil
  s = s - 1
  local n = 1 -- node position in heap array
  local p = 2 * n -- left sibling position
  if s > p and t[h[p]] > t[h[p + 1]] then
    p = 2 * n + 1 -- right sibling position
  end
  while s >= p and t[h[p]] < v do -- descend heap?
    h[p], h[n] = h[n], h[p]
    n = p
    p = 2 * n
    if s > p and t[h[p]] > t[h[p + 1]] then
      p = 2 * n + 1
    end
  end
  return e, r
end

local function isempty (h) return h.heap[1] == nil end

function M.new ()
  return setmetatable({heap = {}, nodes = {}},
      {__index = {push=push, peek=peek, pop=pop, isempty=isempty}})
end

return M