ww: select sources by Go platform suffixes

internal/wwpackage/package.ww

@@ -582,9 +582,108 @@ fn pkgisreg(path: str) bool = {
 	};
 };
 
+fn pkgnamerangeis(name: str, start: i32, end: i32, word: str) bool = {
+	if (end < start || end - start != word.len) { return false; };
+	let i: i32 = 0;
+	for (i < end - start) {
+		if (name[start + i] != word[i]) { return false; };
+		i += 1;
+	};
+	return true;
+};
+
+fn pkgknownos(name: str, start: i32, end: i32) bool = {
+	return pkgnamerangeis(name, start, end, "aix")
+		|| pkgnamerangeis(name, start, end, "android")
+		|| pkgnamerangeis(name, start, end, "darwin")
+		|| pkgnamerangeis(name, start, end, "dragonfly")
+		|| pkgnamerangeis(name, start, end, "freebsd")
+		|| pkgnamerangeis(name, start, end, "hurd")
+		|| pkgnamerangeis(name, start, end, "illumos")
+		|| pkgnamerangeis(name, start, end, "ios")
+		|| pkgnamerangeis(name, start, end, "js")
+		|| pkgnamerangeis(name, start, end, "linux")
+		|| pkgnamerangeis(name, start, end, "nacl")
+		|| pkgnamerangeis(name, start, end, "netbsd")
+		|| pkgnamerangeis(name, start, end, "openbsd")
+		|| pkgnamerangeis(name, start, end, "plan9")
+		|| pkgnamerangeis(name, start, end, "solaris")
+		|| pkgnamerangeis(name, start, end, "wasip1")
+		|| pkgnamerangeis(name, start, end, "windows")
+		|| pkgnamerangeis(name, start, end, "zos");
+};
+
+fn pkgknownarch(name: str, start: i32, end: i32) bool = {
+	return pkgnamerangeis(name, start, end, "386")
+		|| pkgnamerangeis(name, start, end, "amd64")
+		|| pkgnamerangeis(name, start, end, "amd64p32")
+		|| pkgnamerangeis(name, start, end, "arm")
+		|| pkgnamerangeis(name, start, end, "armbe")
+		|| pkgnamerangeis(name, start, end, "arm64")
+		|| pkgnamerangeis(name, start, end, "arm64be")
+		|| pkgnamerangeis(name, start, end, "loong64")
+		|| pkgnamerangeis(name, start, end, "mips")
+		|| pkgnamerangeis(name, start, end, "mipsle")
+		|| pkgnamerangeis(name, start, end, "mips64")
+		|| pkgnamerangeis(name, start, end, "mips64le")
+		|| pkgnamerangeis(name, start, end, "mips64p32")
+		|| pkgnamerangeis(name, start, end, "mips64p32le")
+		|| pkgnamerangeis(name, start, end, "ppc")
+		|| pkgnamerangeis(name, start, end, "ppc64")
+		|| pkgnamerangeis(name, start, end, "ppc64le")
+		|| pkgnamerangeis(name, start, end, "riscv")
+		|| pkgnamerangeis(name, start, end, "riscv64")
+		|| pkgnamerangeis(name, start, end, "s390")
+		|| pkgnamerangeis(name, start, end, "s390x")
+		|| pkgnamerangeis(name, start, end, "sparc")
+		|| pkgnamerangeis(name, start, end, "sparc64")
+		|| pkgnamerangeis(name, start, end, "wasm");
+};
+
+fn pkgtargettag(name: str, start: i32, end: i32) bool = {
+	return pkgnamerangeis(name, start, end, "linux")
+		|| pkgnamerangeis(name, start, end, "amd64");
+};
+
+// Recursive discovery must decide platform eligibility before a source can
+// enter any production or test package graph.
+fn pkgsourceplatform(name: str) bool = {
+	let stem: i32 = 0;
+	let hasunderscore: bool = false;
+	for (stem < name.len && name[stem] != '.') {
+		if (name[stem] == '_') { hasunderscore = true; };
+		stem += 1;
+	};
+	if (!hasunderscore) { return true; };
+
+	let end: i32 = stem;
+	let last: i32 = end;
+	for (last > 0 && name[last - 1] != '_') { last -= 1; };
+	if (pkgnamerangeis(name, last, end, "test")) {
+		if (last == 0) { return true; };
+		end = last - 1;
+		last = end;
+		for (last > 0 && name[last - 1] != '_') { last -= 1; };
+	};
+	if (last == 0) { return true; };
+
+	let prevend: i32 = last - 1;
+	let prev: i32 = prevend;
+	for (prev > 0 && name[prev - 1] != '_') { prev -= 1; };
+	if (prev < prevend && pkgknownos(name, prev, prevend)
+		&& pkgknownarch(name, last, end)) {
+		return pkgtargettag(name, last, end)
+			&& pkgtargettag(name, prev, prevend);
+	};
+	if (pkgknownos(name, last, end) || pkgknownarch(name, last, end)) {
+		return pkgtargettag(name, last, end);
+	};
+	return true;
+};
+
 fn pkgkeepfile(name: str) bool = {
 	if (name.len <= 3 || name[0] == '.' || name[0] == '_') { return false; };
-	return strings.hassuffix(name, ".ww");
+	return strings.hassuffix(name, ".ww") && pkgsourceplatform(name);
 };
 
 type pkgmatcher = struct {