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let bychunk() read the last element; guard for negative sz

I mentioned on IRC that bychunk() was skipping the last element of
a slice, here's a fix for that, as well as some redundant tests and
some probably unneeded input validation. I don't have any strong
feelings about protecting someone from shooting themselves in the
foot with bychunk([1][:], -100), but it's easier to remove in review
than to add later.

--- a/lib/iter/chunk.myr

+++ b/lib/iter/chunk.myr

@@ -12,7 +12,7 @@

 ;;

 generic bychunk = {a, sz

-	-> [.sl = a, .idx = 0, .blksz = sz]

+	-> [.sl = a, .idx = 0, .blksz = std.max(sz, 1)]

 }

 impl iterable chunkiter(@a) -> @a[:] =

@@ -20,7 +20,7 @@

 		var len

 		len = std.min(itp.blksz, itp.sl.len - itp.idx)

-		if itp.idx + len == itp.sl.len

+		if itp.idx + len > itp.sl.len || itp.idx >= itp.sl.len

 			-> false

 		;;

 		valp# = itp.sl[itp.idx: itp.idx + len]

--- /dev/null

+++ b/lib/iter/test/chunk.myr

@@ -1,0 +1,44 @@

+use std

+use testr

+

+use iter

+

+const main = {

+	testr.run([

+		[.name = "bychunk-01", .fn = bychunk01],

+		[.name = "bychunk-02", .fn = bychunk02],

+		[.name = "bychunk-03", .fn = bychunk03],

+		[.name = "bychunk-04", .fn = bychunk04],

+		[.name = "bychunk-05", .fn = bychunk05],

+	][:])

+}

+

+const verify_bychunk = {c, v, n, e

+	var sb : std.strbuf# = std.mksb()

+	std.sbfmt(sb, " ")

+	for w : iter.bychunk(v, n)

+		std.sbfmt(sb, "{} ", w)

+	;;

+	var a : byte[:] = std.sbfin(sb)

+	testr.check(c, std.eq(e, a), "expected “{}”, got “{}”", e, a)

+}

+

+const bychunk01 = {c

+	verify_bychunk(c, [1, 2, 3, 4][:], 1, " [1] [2] [3] [4] ")

+}

+

+const bychunk02 = {c

+	verify_bychunk(c, [1, 2, 3, 4, 5, 6, 7, 8][:], 3, " [1, 2, 3] [4, 5, 6] [7, 8] ")

+}

+

+const bychunk03 = {c

+	verify_bychunk(c, [1, 2, 3, 4][:], 99, " [1, 2, 3, 4] ")

+}

+

+const bychunk04 = {c

+	verify_bychunk(c, [1, 2, 3, 4][:], -2, " [1] [2] [3] [4] ")

+}

+

+const bychunk05 = {c

+	verify_bychunk(c, [][:], 3, " ")

+}