Iterating Over Oddities – of strings, arrays, and counting from 0

Could there possibly be anything more said about iterators and strings? Well, yah, actually tons. Last time around, I showed a simple iterator over a “string”. The focus was primarily on satisfying the job of parsing out null terminated strings from within a ‘null terminated’ string.

As I was doing that, I was also speculating as to whether I could use the exact same iterator to parse fixed sized records from an array of records. I actually wrote a giant iterator that does just that. Then I got to thinking, ‘this isn’t the way to do it’. Iterators, you see, can be broken down into constituent parts. The Lua documentation itself has quite a lot to say about iterators of various forms. One of the most promising bits of documentation is on 7.3 – Stateless Iterators.  In order to pull this off, you split up the “iterator” into a few parts.


generator – The function that gets called every time you need a new value.  The parameters that are passed to it are the “fixedpart”, and the “control”.

invariant state – this is the part of the iterator that doesn’t change much.  For example, the source string that you might be iterating over.

index – this is the part that changes every time generator is called.

so, when you do the following:

local values = {'a', 'b', 'c'}
for _idx, value in ipairs(values) do

The ‘generator’ is some function returned from the ‘ipairs()’ function. It will be called again and again, until it returns nil.

The ‘invariant state’ is the ‘values’ array. This will be fed to the generator each time a new value is needed.

And last, the _idx, is the index value. It will also be fed to the generator, along with the ‘invariant state’.

So, how about applying this to my previous multi string iterator?

local ffi = require("ffi")
local fun = require("fun")

local floor = math.floor;

-- a nil generator.  
-- good for cases when there's no data
local function nil_gen(param, state)
    return nil

local function delim_gen(param, idx)
	local len = 0;

	while ((idx+len) < param.nelems) do
		--print("wchar: ", string.char(ffi.cast(param.basetypeptr,[idx + len]))
		if ffi.cast(param.basetypeptr,[idx + len] ~= param.separator then
			len = len + 1;
	if len == 0 then
		return nil;

	return idx + len + 1, ffi.cast(param.basetypeptr,, len

local function array_gen(param, idx)
	if idx >= param.nelems then
		return nil;

	return idx+1, ffi.cast(param.basetypeptr,, 1

local function striter(params)
	if not params then
		return nil_gen, params, nil

	if not then
		return nil_gen, params, nil

	params.datalength = params.datalength or
	if params.basetype then
		if type(params.basetype)== "string" then
			params.basetype = ffi.typeof(params.basetype)
	params.basetype = params.basetype or ffi.typeof("char")
	params.basetypeptr = ffi.typeof("const $ *", params.basetype)
	params.basetypesize = ffi.sizeof(params.basetype)
	params.nelems = math.floor(params.datalength / params.basetypesize)

	if params.separator ~= nil then
		return delim_gen, params, 0
		return array_gen, params, 0

	return nil_gen, nil, nil

How to apply it?

local src3 = "big,boy,baby,bear,bounces,basketballs,behind,the,barn,,"

local function printAnsi(ptr, len)
  print(ffi.string(ptr, len))

each(printAnsi, striter{data=src3, basetype="char"})

Here, I am using the Lua Fun ‘each’ function to drive my iterator. I could just as easily use a simple ‘for – in’ loop, but I’m getting all functional these days. What the last statement says is, “for each of the items coming out of the striter iterator, call the ‘printAnsi()’ function”.

The striter function is called with a table that contains the various parameters it will need. In this particular case, I’ve left off the parenthesis, because in Lua, if it’s just a single table value, or a string, you can do that.

So, how about that striter() function? Looking back, it has the job of returning a ‘generator’, ‘invariant state’, and a ‘index’. Well, it cheats a bit because the ‘invariant’ also contains the ‘index’. The ‘invariant’ is the fact that the table value doesn’t change, even though the contents can. This is just a matter of convenience.

At any rate, the striter() function decides which generator to return based on what it sees in the parameters. For example, if it sees a separator, then it will return the ‘delim_gen’ generator. That generator functions pretty much the same was as the one I created last time for the multi string thing. In the case where it doesn’t see a separator, it will return the ‘array_gen’ generator. That generator will assume it is being handed a pointer to an array of values of a particular type.

One thing to note that is different this time around from the mstrziter, Lua string creation does not occur within the iterator itself. Rather than return a string value, the iterator will simply return an offset and a length. It is up to the caller to determine what they want to do with the values.

This is kind of a key to an IEnumerable chain. Do the least amount of work as possibly, deferring really heavy work towards the end of your chain. This lazy evaluation makes for a more efficient chain. So, the ‘printAnsi’ function is at the end of the chain. It might have turned out that instead of creating strings at all, I might have wanted to send the values across a network to be stored in a database. In that case, the pointer, offset, length is perfect to be consumed directy by the Socket:send(buff, len) function, so no copying would be necessary.

How about that array case?

Let’s imagine I wanted to print out every value of the string one by one.

each(printAnsi, striter{data=src3, basetype="char"})

In this case, I’m creating an iterator, not specifying the separator (so the array_gen will be used). I have also specified the ‘basetype’ of the elements of my array. That’s so it can calculate how many there are, and create a pointer of the appropriate type. And you’re done!

Of course, the ‘basetype’ could just as easily be ‘BGR32’, or ‘PersonRecord’, or whatever fixed size type you so happen to have stored in some array. Makes for some fairly easy ‘tokenizing’ of array values.

To go further, what’s say you have a multi string based on ‘wchar_t’, and delimeted by ‘ ‘ (space) characters?

How about a little convenience function?

local function wmstriter(data, separator, datalength)
  if type(separator) == "string" then
    separator = string.byte(separator)

  datalength = datalength or ffi.sizeof(data)
  return map(core_string.toAnsi, striter{data=data, datalength = datalength, basetype="wchar_t", separator=separator})

and using it:

local whello, whellolen = core_string.toUnicode("Hello World");

each(print, wmstriter(whello, ' '))

In this case, I create a ‘wchar_t’ based string, using the ‘toUnicode()’ function. Then I feed that to the wmstriter iterator, and print out each of the words, delimeted by a ‘(sp)’.

The core ‘striter()’ remains the same, and the generators don’t change. You just build up more useful ways of feeding them and consuming them.

I find this to be fairly useful and powerful. When you think about the generators, they have a fairly simple task. Do one small job, and do it well. The complexity of a larger system is gained through thoughtful composition of these simpler parts, rather than building hugely complex macro parts.

This is helpful for code maintenance. I find it hard to maintain largish single functions with hundreds of lines of code. I find it fairly easy to maintain a generator that has a few lines of code, and does a fairly simple job. I find the composition model easy as well. I can look back on it over time, read the functional chain, and understand what was intended.

And there you have it. More iterating over oddities.


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