local lib={}
lib.internal={}

local function checkArg(n, have, ...)
	have = type(have)
	local function check(want, ...)
		if not want then
			return false
		else
			return have == want or check(...)
		end
	end
	if not check(...) then
		local msg = string.format("bad argument #%d (%s expected, got %s)",
															n, table.concat({...}, " or "), have)
		error(msg, 3)
	end
end

function lib.internal.range_adjust(f,l,s)
	checkArg(1,f,'number','nil')
	checkArg(2,l,'number','nil')
	checkArg(3,s,'number')
	if f==nil then f=1 elseif f<0 then f=s+f+1 end
	if l==nil then l=s elseif l<0 then l=s+l+1 end
	return f,l
end
function lib.internal.table_view(tbl,f,l)
	return setmetatable({},
	{
		__index = function(_, key)
			return (type(key) ~= 'number' or (key >= f and key <= l)) and tbl[key] or nil
		end,
		__len = function(_)
			return l
		end,
	})
end
local adjust=lib.internal.range_adjust
local view=lib.internal.table_view

-- first(p1,p2) searches for the first range in p1 that satisfies p2
function lib.first(tbl,pred,f,l)
	checkArg(1,tbl,'table')
	checkArg(2,pred,'function','table')
	if type(pred)=='table'then
		local set;set,pred=pred,function(_,fi,tbl)
			for vi=1,#set do
				local v=set[vi]
				if lib.begins(tbl,v,fi) then return true,#v end
			end
		end
	end
	local s=#tbl
	f,l=adjust(f,l,s)
	tbl=view(tbl,f,l)
	for i=f,l do
		local si,ei=pred(tbl[i],i,tbl)
		if si then
			return i,i+(ei or 1)-1
		end
	end
end

-- returns true if p1 at first p3 equals element for element p2
function lib.begins(tbl,v,f,l)
	checkArg(1,tbl,'table')
	checkArg(2,v,'table')
	local vs=#v
	f,l=adjust(f,l,#tbl)
	if vs>(l-f+1)then return end
	for i=1,vs do
		if tbl[f+i-1]~=v[i] then return end
	end
	return true
end

function lib.concat(...)
	local r,rn,k={},0
	for _,tbl in ipairs({...})do
		if type(tbl)~='table'then
			return nil,'parameter '..tostring(_)..' to concat is not a table'
		end
		local n=tbl.n or #tbl
		k=k or tbl.n
		for i=1,n do
			rn=rn+1;r[rn]=tbl[i]
		end
	end
	r.n=k and rn or nil
	return r
end

-- works like string.sub but on elements of an indexed table
function lib.sub(tbl,f,l)
	checkArg(1,tbl,'table')
	local r,s={},#tbl
	f,l=adjust(f,l,s)
	l=math.min(l,s)
	for i=math.max(f,1),l do
		r[#r+1]=tbl[i]
	end
	return r
end

-- Returns a list of subsets of tbl where partitioner acts as a delimiter.
function lib.partition(tbl,partitioner,dropEnds,f,l)
	checkArg(1,tbl,'table')
	checkArg(2,partitioner,'function','table')
	checkArg(3,dropEnds,'boolean','nil')
	if type(partitioner)=='table'then
		return lib.partition(tbl,function(_,i,tbl)
			return lib.first(tbl,partitioner,i)
		end,dropEnds,f,l)
	end
	local s=#tbl
	f,l=adjust(f,l,s)
	local cut=view(tbl,f,l)
	local result={}
	local need=true
	local exp=function()if need then result[#result+1]={}need=false end end
	local i=f
	while i<=l do
		local e=cut[i]
		local ds,de=partitioner(e,i,cut)
		-- true==partition here
		if ds==true then ds,de=i,i
		elseif ds==false then ds,de=nil,nil end
		if ds~=nil then
			ds,de=adjust(ds,de,l)
			ds=ds>=i and ds--no more
		end
		if not ds then -- false or nil
			exp()
			table.insert(result[#result],e)
		else
			local sub=lib.sub(cut,i,not dropEnds and de or (ds-1))
			if #sub>0 then
				exp()
				result[#result+math.min(#result[#result],1)]=sub
			end
			-- ensure i moves forward
			local ensured=math.max(math.max(de or ds,ds),i)
			if de and ds and de<ds and ensured==i then
				if #result==0 then result[1]={} end
				table.insert(result[#result],e)
			end
			i=ensured
			need=true
		end
		i=i+1
	end

	return result
end

-- calls callback(e,i,tbl) for each ith element e in table tbl from first
function lib.foreach(tbl,c,f,l)
	checkArg(1,tbl,'table')
	checkArg(2,c,'function','string')
	local ck=c
	c=type(c)=="string" and function(e) return e[ck] end or c
	local s=#tbl
	f,l=adjust(f,l,s)
	tbl=view(tbl,f,l)
	local r={}
	for i=f,l do
		local n,k=c(tbl[i],i,tbl)
		if n~=nil then
			if k then r[k]=n
			else r[#r+1]=n end
		end
	end
	return r
end

function lib.where(tbl,p,f,l)
	return lib.foreach(tbl,
		function(e,i,tbl)
			return p(e,i,tbl)and e or nil
		end,f,l)
end

-- works with pairs on tables
-- returns the kv pair, or nil and the number of pairs iterated
function lib.at(tbl, index)
	checkArg(1, tbl, "table")
	checkArg(2, index, "number", "nil")
	local current_index = 1
	for k,v in pairs(tbl) do
		if current_index == index then
			return k,v
		end
		current_index = current_index + 1
	end
	return nil, current_index - 1 -- went one too far
end

return lib