---@defgroup vim.iter
---
--- \*vim.iter()\* is an interface for |iterable|s: it wraps a table or function argument into an
--- \*Iter\* object with methods (such as |Iter:filter()| and |Iter:map()|) that transform the
--- underlying source data. These methods can be chained to create iterator "pipelines": the output
--- of each pipeline stage is input to the next stage. The first stage depends on the type passed to
--- `vim.iter()`:
---
--- - List tables (arrays, |lua-list|) yield only the value of each element.
---   - Use |Iter:enumerate()| to also pass the index to the next stage.
---   - Or initialize with ipairs(): `vim.iter(ipairs(…))`.
--- - Non-list tables (|lua-dict|) yield both the key and value of each element.
--- - Function |iterator|s yield all values returned by the underlying function.
--- - Tables with a |__call()| metamethod are treated as function iterators.
---
--- The iterator pipeline terminates when the underlying |iterable| is exhausted (for function
--- iterators this means it returned nil).
---
--- Note: `vim.iter()` scans table input to decide if it is a list or a dict; to avoid this cost you
--- can wrap the table with an iterator e.g. `vim.iter(ipairs({…}))`, but that precludes the use of
--- |list-iterator| operations such as |Iter:rev()|).
---
--- Examples:
---
--- ```lua
--- local it = vim.iter({ 1, 2, 3, 4, 5 })
--- it:map(function(v)
---   return v * 3
--- end)
--- it:rev()
--- it:skip(2)
--- it:totable()
--- -- { 9, 6, 3 }
---
--- -- ipairs() is a function iterator which returns both the index (i) and the value (v)
--- vim.iter(ipairs({ 1, 2, 3, 4, 5 })):map(function(i, v)
---   if i > 2 then return v end
--- end):totable()
--- -- { 3, 4, 5 }
---
--- local it = vim.iter(vim.gsplit('1,2,3,4,5', ','))
--- it:map(function(s) return tonumber(s) end)
--- for i, d in it:enumerate() do
---   print(string.format("Column %d is %d", i, d))
--- end
--- -- Column 1 is 1
--- -- Column 2 is 2
--- -- Column 3 is 3
--- -- Column 4 is 4
--- -- Column 5 is 5
---
--- vim.iter({ a = 1, b = 2, c = 3, z = 26 }):any(function(k, v)
---   return k == 'z'
--- end)
--- -- true
---
--- local rb = vim.ringbuf(3)
--- rb:push("a")
--- rb:push("b")
--- vim.iter(rb):totable()
--- -- { "a", "b" }
--- ```
---
--- In addition to the |vim.iter()| function, the |vim.iter| module provides
--- convenience functions like |vim.iter.filter()| and |vim.iter.totable()|.

---@class IterMod
---@operator call:Iter
local M = {}

---@class Iter
local Iter = {}
Iter.__index = Iter
Iter.__call = function(self)
  return self:next()
end

--- Special case implementations for iterators on list tables.
---@class ListIter : Iter
---@field _table table Underlying table data
---@field _head number Index to the front of a table iterator
---@field _tail number Index to the end of a table iterator (exclusive)
local ListIter = {}
ListIter.__index = setmetatable(ListIter, Iter)
ListIter.__call = function(self)
  return self:next()
end

--- Packed tables use this as their metatable
local packedmt = {}

local function unpack(t)
  if type(t) == 'table' and getmetatable(t) == packedmt then
    return _G.unpack(t, 1, t.n)
  end
  return t
end

local function pack(...)
  local n = select('#', ...)
  if n > 1 then
    return setmetatable({ n = n, ... }, packedmt)
  end
  return ...
end

local function sanitize(t)
  if type(t) == 'table' and getmetatable(t) == packedmt then
    -- Remove length tag
    t.n = nil
  end
  return t
end

--- Determine if the current iterator stage should continue.
---
--- If any arguments are passed to this function, then return those arguments
--- and stop the current iterator stage. Otherwise, return true to signal that
--- the current stage should continue.
---
---@param ... any Function arguments.
---@return boolean True if the iterator stage should continue, false otherwise
---@return any Function arguments.
local function continue(...)
  if select(1, ...) ~= nil then
    return false, ...
  end
  return true
end

--- If no input arguments are given return false, indicating the current
--- iterator stage should stop. Otherwise, apply the arguments to the function
--- f. If that function returns no values, the current iterator stage continues.
--- Otherwise, those values are returned.
---
---@param f function Function to call with the given arguments
---@param ... any Arguments to apply to f
---@return boolean True if the iterator pipeline should continue, false otherwise
---@return any Return values of f
local function apply(f, ...)
  if select(1, ...) ~= nil then
    return continue(f(...))
  end
  return false
end

--- Filters an iterator pipeline.
---
--- Example:
---
--- ```lua
--- local bufs = vim.iter(vim.api.nvim_list_bufs()):filter(vim.api.nvim_buf_is_loaded)
--- ```
---
---@param f function(...):bool Takes all values returned from the previous stage
---                            in the pipeline and returns false or nil if the
---                            current iterator element should be removed.
---@return Iter
function Iter.filter(self, f)
  return self:map(function(...)
    if f(...) then
      return ...
    end
  end)
end

---@private
function ListIter.filter(self, f)
  local inc = self._head < self._tail and 1 or -1
  local n = self._head
  for i = self._head, self._tail - inc, inc do
    local v = self._table[i]
    if f(unpack(v)) then
      self._table[n] = v
      n = n + inc
    end
  end
  self._tail = n
  return self
end

--- Maps the items of an iterator pipeline to the values returned by `f`.
---
--- If the map function returns nil, the value is filtered from the iterator.
---
--- Example:
---
--- ```lua
--- local it = vim.iter({ 1, 2, 3, 4 }):map(function(v)
---   if v % 2 == 0 then
---     return v * 3
---   end
--- end)
--- it:totable()
--- -- { 6, 12 }
--- ```
---
---@param f function(...):any Mapping function. Takes all values returned from
---                           the previous stage in the pipeline as arguments
---                           and returns one or more new values, which are used
---                           in the next pipeline stage. Nil return values
---                           are filtered from the output.
---@return Iter
function Iter.map(self, f)
  -- Implementation note: the reader may be forgiven for observing that this
  -- function appears excessively convoluted. The problem to solve is that each
  -- stage of the iterator pipeline can return any number of values, and the
  -- number of values could even change per iteration. And the return values
  -- must be checked to determine if the pipeline has ended, so we cannot
  -- naively forward them along to the next stage.
  --
  -- A simple approach is to pack all of the return values into a table, check
  -- for nil, then unpack the table for the next stage. However, packing and
  -- unpacking tables is quite slow. There is no other way in Lua to handle an
  -- unknown number of function return values than to simply forward those
  -- values along to another function. Hence the intricate function passing you
  -- see here.

  local next = self.next

  --- Drain values from the upstream iterator source until a value can be
  --- returned.
  ---
  --- This is a recursive function. The base case is when the first argument is
  --- false, which indicates that the rest of the arguments should be returned
  --- as the values for the current iteration stage.
  ---
  ---@param cont boolean If true, the current iterator stage should continue to
  ---                    pull values from its upstream pipeline stage.
  ---                    Otherwise, this stage is complete and returns the
  ---                    values passed.
  ---@param ... any Values to return if cont is false.
  ---@return any
  local function fn(cont, ...)
    if cont then
      return fn(apply(f, next(self)))
    end
    return ...
  end

  self.next = function()
    return fn(apply(f, next(self)))
  end
  return self
end

---@private
function ListIter.map(self, f)
  local inc = self._head < self._tail and 1 or -1
  local n = self._head
  for i = self._head, self._tail - inc, inc do
    local v = pack(f(unpack(self._table[i])))
    if v ~= nil then
      self._table[n] = v
      n = n + inc
    end
  end
  self._tail = n
  return self
end

--- Calls a function once for each item in the pipeline, draining the iterator.
---
--- For functions with side effects. To modify the values in the iterator, use |Iter:map()|.
---
---@param f function(...) Function to execute for each item in the pipeline.
---                       Takes all of the values returned by the previous stage
---                       in the pipeline as arguments.
function Iter.each(self, f)
  local function fn(...)
    if select(1, ...) ~= nil then
      f(...)
      return true
    end
  end
  while fn(self:next()) do
  end
end

---@private
function ListIter.each(self, f)
  local inc = self._head < self._tail and 1 or -1
  for i = self._head, self._tail - inc, inc do
    f(unpack(self._table[i]))
  end
  self._head = self._tail
end

--- Collect the iterator into a table.
---
--- The resulting table depends on the initial source in the iterator pipeline.
--- List-like tables and function iterators will be collected into a list-like
--- table. If multiple values are returned from the final stage in the iterator
--- pipeline, each value will be included in a table.
---
--- Examples:
---
--- ```lua
--- vim.iter(string.gmatch('100 20 50', '%d+')):map(tonumber):totable()
--- -- { 100, 20, 50 }
---
--- vim.iter({ 1, 2, 3 }):map(function(v) return v, 2 * v end):totable()
--- -- { { 1, 2 }, { 2, 4 }, { 3, 6 } }
---
--- vim.iter({ a = 1, b = 2, c = 3 }):filter(function(k, v) return v % 2 ~= 0 end):totable()
--- -- { { 'a', 1 }, { 'c', 3 } }
--- ```
---
--- The generated table is a list-like table with consecutive, numeric indices.
--- To create a map-like table with arbitrary keys, use |Iter:fold()|.
---
---
---@return table
function Iter.totable(self)
  local t = {}

  while true do
    local args = pack(self:next())
    if args == nil then
      break
    end

    t[#t + 1] = sanitize(args)
  end
  return t
end

---@private
function ListIter.totable(self)
  if self.next ~= ListIter.next or self._head >= self._tail then
    return Iter.totable(self)
  end

  local needs_sanitize = getmetatable(self._table[1]) == packedmt

  -- Reindex and sanitize.
  local len = self._tail - self._head

  if needs_sanitize then
    for i = 1, len do
      self._table[i] = sanitize(self._table[self._head - 1 + i])
    end
  else
    for i = 1, len do
      self._table[i] = self._table[self._head - 1 + i]
    end
  end

  for i = len + 1, table.maxn(self._table) do
    self._table[i] = nil
  end

  self._head = 1
  self._tail = len + 1

  return self._table
end

--- Folds ("reduces") an iterator into a single value.
---
--- Examples:
---
--- ```lua
--- -- Create a new table with only even values
--- local t = { a = 1, b = 2, c = 3, d = 4 }
--- local it = vim.iter(t)
--- it:filter(function(k, v) return v % 2 == 0 end)
--- it:fold({}, function(t, k, v)
---   t[k] = v
---   return t
--- end)
--- -- { b = 2, d = 4 }
--- ```
---
---@generic A
---
---@param init A Initial value of the accumulator.
---@param f function(acc:A, ...):A Accumulation function.
---@return A
function Iter.fold(self, init, f)
  local acc = init

  --- Use a closure to handle var args returned from iterator
  local function fn(...)
    if select(1, ...) ~= nil then
      acc = f(acc, ...)
      return true
    end
  end

  while fn(self:next()) do
  end
  return acc
end

---@private
function ListIter.fold(self, init, f)
  local acc = init
  local inc = self._head < self._tail and 1 or -1
  for i = self._head, self._tail - inc, inc do
    acc = f(acc, unpack(self._table[i]))
  end
  return acc
end

--- Gets the next value from the iterator.
---
--- Example:
---
--- ```lua
---
--- local it = vim.iter(string.gmatch('1 2 3', '%d+')):map(tonumber)
--- it:next()
--- -- 1
--- it:next()
--- -- 2
--- it:next()
--- -- 3
---
--- ```
---
---@return any
function Iter.next(self) -- luacheck: no unused args
  -- This function is provided by the source iterator in Iter.new. This definition exists only for
  -- the docstring
end

---@private
function ListIter.next(self)
  if self._head ~= self._tail then
    local v = self._table[self._head]
    local inc = self._head < self._tail and 1 or -1
    self._head = self._head + inc
    return unpack(v)
  end
end

--- Reverses a |list-iterator| pipeline.
---
--- Example:
---
--- ```lua
---
--- local it = vim.iter({ 3, 6, 9, 12 }):rev()
--- it:totable()
--- -- { 12, 9, 6, 3 }
---
--- ```
---
---@return Iter
function Iter.rev(self)
  error('rev() requires a list-like table')
  return self
end

---@private
function ListIter.rev(self)
  local inc = self._head < self._tail and 1 or -1
  self._head, self._tail = self._tail - inc, self._head - inc
  return self
end

--- Gets the next value in a |list-iterator| without consuming it.
---
--- Example:
---
--- ```lua
---
--- local it = vim.iter({ 3, 6, 9, 12 })
--- it:peek()
--- -- 3
--- it:peek()
--- -- 3
--- it:next()
--- -- 3
---
--- ```
---
---@return any
function Iter.peek(self) -- luacheck: no unused args
  error('peek() requires a list-like table')
end

---@private
function ListIter.peek(self)
  if self._head ~= self._tail then
    return self._table[self._head]
  end
end

--- Find the first value in the iterator that satisfies the given predicate.
---
--- Advances the iterator. Returns nil and drains the iterator if no value is found.
---
--- Examples:
---
--- ```lua
---
--- local it = vim.iter({ 3, 6, 9, 12 })
--- it:find(12)
--- -- 12
---
--- local it = vim.iter({ 3, 6, 9, 12 })
--- it:find(20)
--- -- nil
---
--- local it = vim.iter({ 3, 6, 9, 12 })
--- it:find(function(v) return v % 4 == 0 end)
--- -- 12
---
--- ```
---
---@return any
function Iter.find(self, f)
  if type(f) ~= 'function' then
    local val = f
    f = function(v)
      return v == val
    end
  end

  local result = nil

  --- Use a closure to handle var args returned from iterator
  local function fn(...)
    if select(1, ...) ~= nil then
      if f(...) then
        result = pack(...)
      else
        return true
      end
    end
  end

  while fn(self:next()) do
  end
  return unpack(result)
end

--- Gets the first value in a |list-iterator| that satisfies a predicate, starting from the end.
---
--- Advances the iterator. Returns nil and drains the iterator if no value is found.
---
--- Examples:
---
--- ```lua
---
--- local it = vim.iter({ 1, 2, 3, 2, 1 }):enumerate()
--- it:rfind(1)
--- -- 5	1
--- it:rfind(1)
--- -- 1	1
---
--- ```
---
---@see Iter.find
---
---@return any
function Iter.rfind(self, f) -- luacheck: no unused args
  error('rfind() requires a list-like table')
end

---@private
function ListIter.rfind(self, f) -- luacheck: no unused args
  if type(f) ~= 'function' then
    local val = f
    f = function(v)
      return v == val
    end
  end

  local inc = self._head < self._tail and 1 or -1
  for i = self._tail - inc, self._head, -inc do
    local v = self._table[i]
    if f(unpack(v)) then
      self._tail = i
      return unpack(v)
    end
  end
  self._head = self._tail
end

--- "Pops" a value from a |list-iterator| (gets the last value and decrements the tail).
---
--- Example:
---
--- ```lua
--- local it = vim.iter({1, 2, 3, 4})
--- it:nextback()
--- -- 4
--- it:nextback()
--- -- 3
--- ```
---
---@return any
function Iter.nextback(self) -- luacheck: no unused args
  error('nextback() requires a list-like table')
end

function ListIter.nextback(self)
  if self._head ~= self._tail then
    local inc = self._head < self._tail and 1 or -1
    self._tail = self._tail - inc
    return self._table[self._tail]
  end
end

--- Gets the last value of a |list-iterator| without consuming it.
---
--- See also |Iter:last()|.
---
--- Example:
---
--- ```lua
--- local it = vim.iter({1, 2, 3, 4})
--- it:peekback()
--- -- 4
--- it:peekback()
--- -- 4
--- it:nextback()
--- -- 4
--- ```
---
---@return any
function Iter.peekback(self) -- luacheck: no unused args
  error('peekback() requires a list-like table')
end

function ListIter.peekback(self)
  if self._head ~= self._tail then
    local inc = self._head < self._tail and 1 or -1
    return self._table[self._tail - inc]
  end
end

--- Skips `n` values of an iterator pipeline.
---
--- Example:
---
--- ```lua
---
--- local it = vim.iter({ 3, 6, 9, 12 }):skip(2)
--- it:next()
--- -- 9
---
--- ```
---
---@param n number Number of values to skip.
---@return Iter
function Iter.skip(self, n)
  for _ = 1, n do
    local _ = self:next()
  end
  return self
end

---@private
function ListIter.skip(self, n)
  local inc = self._head < self._tail and n or -n
  self._head = self._head + inc
  if (inc > 0 and self._head > self._tail) or (inc < 0 and self._head < self._tail) then
    self._head = self._tail
  end
  return self
end

--- Skips `n` values backwards from the end of a |list-iterator| pipeline.
---
--- Example:
---
--- ```lua
--- local it = vim.iter({ 1, 2, 3, 4, 5 }):skipback(2)
--- it:next()
--- -- 1
--- it:nextback()
--- -- 3
--- ```
---
---@param n number Number of values to skip.
---@return Iter
function Iter.skipback(self, n) -- luacheck: no unused args
  error('skipback() requires a list-like table')
  return self
end

---@private
function ListIter.skipback(self, n)
  local inc = self._head < self._tail and n or -n
  self._tail = self._tail - inc
  if (inc > 0 and self._head > self._tail) or (inc < 0 and self._head < self._tail) then
    self._head = self._tail
  end
  return self
end

--- Gets the nth value of an iterator (and advances to it).
---
--- Example:
---
--- ```lua
---
--- local it = vim.iter({ 3, 6, 9, 12 })
--- it:nth(2)
--- -- 6
--- it:nth(2)
--- -- 12
---
--- ```
---
---@param n number The index of the value to return.
---@return any
function Iter.nth(self, n)
  if n > 0 then
    return self:skip(n - 1):next()
  end
end

--- Gets the nth value from the end of a |list-iterator| (and advances to it).
---
--- Example:
---
--- ```lua
---
--- local it = vim.iter({ 3, 6, 9, 12 })
--- it:nthback(2)
--- -- 9
--- it:nthback(2)
--- -- 3
---
--- ```
---
---@param n number The index of the value to return.
---@return any
function Iter.nthback(self, n)
  if n > 0 then
    return self:skipback(n - 1):nextback()
  end
end

--- Sets the start and end of a |list-iterator| pipeline.
---
--- Equivalent to `:skip(first - 1):skipback(len - last + 1)`.
---
---@param first number
---@param last number
---@return Iter
function Iter.slice(self, first, last) -- luacheck: no unused args
  error('slice() requires a list-like table')
  return self
end

---@private
function ListIter.slice(self, first, last)
  return self:skip(math.max(0, first - 1)):skipback(math.max(0, self._tail - last - 1))
end

--- Returns true if any of the items in the iterator match the given predicate.
---
---@param pred function(...):bool Predicate function. Takes all values returned from the previous
---                                stage in the pipeline as arguments and returns true if the
---                                predicate matches.
function Iter.any(self, pred)
  local any = false

  --- Use a closure to handle var args returned from iterator
  local function fn(...)
    if select(1, ...) ~= nil then
      if pred(...) then
        any = true
      else
        return true
      end
    end
  end

  while fn(self:next()) do
  end
  return any
end

--- Returns true if all items in the iterator match the given predicate.
---
---@param pred function(...):bool Predicate function. Takes all values returned from the previous
---                                stage in the pipeline as arguments and returns true if the
---                                predicate matches.
function Iter.all(self, pred)
  local all = true

  local function fn(...)
    if select(1, ...) ~= nil then
      if not pred(...) then
        all = false
      else
        return true
      end
    end
  end

  while fn(self:next()) do
  end
  return all
end

--- Drains the iterator and returns the last item.
---
--- Example:
---
--- ```lua
---
--- local it = vim.iter(vim.gsplit('abcdefg', ''))
--- it:last()
--- -- 'g'
---
--- local it = vim.iter({ 3, 6, 9, 12, 15 })
--- it:last()
--- -- 15
---
--- ```
---
---@return any
function Iter.last(self)
  local last = self:next()
  local cur = self:next()
  while cur do
    last = cur
    cur = self:next()
  end
  return last
end

---@private
function ListIter.last(self)
  local inc = self._head < self._tail and 1 or -1
  local v = self._table[self._tail - inc]
  self._head = self._tail
  return v
end

--- Yields the item index (count) and value for each item of an iterator pipeline.
---
--- For list tables, this is more efficient:
---
--- ```lua
--- vim.iter(ipairs(t))
--- ```
---
--- instead of:
---
--- ```lua
--- vim.iter(t):enumerate()
--- ```
---
--- Example:
---
--- ```lua
---
--- local it = vim.iter(vim.gsplit('abc', '')):enumerate()
--- it:next()
--- -- 1	'a'
--- it:next()
--- -- 2	'b'
--- it:next()
--- -- 3	'c'
---
--- ```
---
---@return Iter
function Iter.enumerate(self)
  local i = 0
  return self:map(function(...)
    i = i + 1
    return i, ...
  end)
end

---@private
function ListIter.enumerate(self)
  local inc = self._head < self._tail and 1 or -1
  for i = self._head, self._tail - inc, inc do
    local v = self._table[i]
    self._table[i] = pack(i, v)
  end
  return self
end

--- Creates a new Iter object from a table or other |iterable|.
---
---@param src table|function Table or iterator to drain values from
---@return Iter
---@private
function Iter.new(src, ...)
  local it = {}
  if type(src) == 'table' then
    local mt = getmetatable(src)
    if mt and type(mt.__call) == 'function' then
      ---@private
      function it.next()
        return src()
      end

      setmetatable(it, Iter)
      return it
    end

    local t = {}

    -- O(n): scan the source table to decide if it is a list (consecutive integer indices 1…n).
    local count = 0
    for _ in pairs(src) do
      count = count + 1
      local v = src[count]
      if v == nil then
        return Iter.new(pairs(src))
      end
      t[count] = v
    end
    return ListIter.new(t)
  end

  if type(src) == 'function' then
    local s, var = ...

    --- Use a closure to handle var args returned from iterator
    local function fn(...)
      -- Per the Lua 5.1 reference manual, an iterator is complete when the first returned value is
      -- nil (even if there are other, non-nil return values). See |for-in|.
      if select(1, ...) ~= nil then
        var = select(1, ...)
        return ...
      end
    end

    ---@private
    function it.next()
      return fn(src(s, var))
    end

    setmetatable(it, Iter)
  else
    error('src must be a table or function')
  end
  return it
end

--- Create a new ListIter
---
---@param t table List-like table. Caller guarantees that this table is a valid list.
---@return Iter
---@private
function ListIter.new(t)
  local it = {}
  it._table = t
  it._head = 1
  it._tail = #t + 1
  setmetatable(it, ListIter)
  return it
end

--- Collects an |iterable| into a table.
---
--- ```lua
--- -- Equivalent to:
--- vim.iter(f):totable()
--- ```
---
---@param f function Iterator function
---@return table
function M.totable(f, ...)
  return Iter.new(f, ...):totable()
end

--- Filters a table or other |iterable|.
---
--- ```lua
--- -- Equivalent to:
--- vim.iter(src):filter(f):totable()
--- ```
---
---@see |Iter:filter()|
---
---@param f function(...):bool Filter function. Accepts the current iterator or table values as
---                            arguments and returns true if those values should be kept in the
---                            final table
---@param src table|function Table or iterator function to filter
---@return table
function M.filter(f, src, ...)
  return Iter.new(src, ...):filter(f):totable()
end

--- Maps a table or other |iterable|.
---
--- ```lua
--- -- Equivalent to:
--- vim.iter(src):map(f):totable()
--- ```
---
---@see |Iter:map()|
---
---@param f function(...):?any Map function. Accepts the current iterator or table values as
---                            arguments and returns one or more new values. Nil values are removed
---                            from the final table.
---@param src table|function Table or iterator function to filter
---@return table
function M.map(f, src, ...)
  return Iter.new(src, ...):map(f):totable()
end

---@type IterMod
return setmetatable(M, {
  __call = function(_, ...)
    return Iter.new(...)
  end,
})