local array = {}

function array.find(arr, elem)
  if type(elem) == 'function' then
    for i,x in ipairs(arr) do
      if elem(x) then
        return x, i
      end
    end
  else
    for i,x in ipairs(arr) do
      if x == elem then
        return i
      end
    end
  end
  return nil
end

function array.any(arr, f)
  for i,x in ipairs(arr) do
    local result = f(x)
    if result then
      return result
    end
  end
  return false
end

function array.all(arr, f)
  for i,x in ipairs(arr) do
    if not f(x) then
      return false
    end
  end
  return true
end

-- like ipairs, but support an index to start iterating from
-- idx is inclusive; the first call to the iterator will return idx
function array.each(arr, idx)
  if idx then
    if type(idx) ~= 'number' then
      error("argument #2 to 'array.each' must be an integer if provided")
    end
    assert(idx >= 1, ("can't start iterating over an array from %d; the starting index needs to be at least 1"):format(idx))
  end
  idx = idx and idx-1 or 0
  local iter = ipairs(arr)
  return function()
    local elem
    idx, elem = iter(arr, idx)
    return idx, elem
  end
end

function array.join(a, b)
  local result = {}
  for _,v in ipairs(a) do table.insert(result, v) end
  for _,v in ipairs(b) do table.insert(result, v) end
  return result
end

function array.map(arr, f)
  local result = {}
  for _,v in ipairs(arr) do table.insert(result, f(v)) end
  return result
end

function array.reduce(arr, init, f)
  local result = init
  for _,v in ipairs(arr) do
    result = f(result, v)
  end
  return result
end

function array.sum(arr, f)
  return array.reduce(array.map(arr, f), 0,
    function(a, b) return a+b end)
end

function array.product(arr, f)
  return array.reduce(array.map(arr, f), 1,
    function(a, b) return a*b end)
end

function array.keep(arr, f)
  local result = {}
  for _,v in ipairs(arr) do
    if f(v) then table.insert(result, v) end end
  return result
end

function array.remove(arr, f)
  local result = {}
  for _,v in ipairs(arr) do
    if not f(v) then table.insert(result, v) end end
  return result
end

return array