-- https://pico-8.fandom.com/wiki/Qsort
function qsort(a,c,l,r)
	c,l,r=c or function(a,b) return a<b end,l or 1,r or #a
	if l<r then
		if c(a[r],a[l]) then
			a[l],a[r]=a[r],a[l]
		end
		local lp,k,rp,p,q=l+1,l+1,r-1,a[l],a[r]
		while k<=rp do
			local swaplp=c(a[k],p)
			-- "if a or b then else"
			-- saves a token versus
			-- "if not (a or b) then"
			if swaplp or c(a[k],q) then
			else
				while c(q,a[rp]) and k<rp do
					rp-=1
				end
				a[k],a[rp],swaplp=a[rp],a[k],c(a[rp],p)
				rp-=1
			end
			if swaplp then
				a[k],a[lp]=a[lp],a[k]
				lp+=1
			end
			k+=1
		end
		lp-=1
		rp+=1
		-- sometimes lp==rp, so 
		-- these two lines *must*
		-- occur in sequence;
		-- don't combine them to
		-- save a token!
		a[l],a[lp]=a[lp],a[l]
		a[r],a[rp]=a[rp],a[r]
		qsort(a,c,l,lp-1       )
		qsort(a,c,  lp+1,rp-1  )
		qsort(a,c,       rp+1,r)
	end
    return a
end

function filter(a, pred)
    filtered = {}
    for k,v in ipairs(a) do
        if pred(v) then
            filtered[k] = v
        end
    end
    return filtered
end