ADDED   src/0dev.org/commands/tbd/tbd.go
Index: src/0dev.org/commands/tbd/tbd.go
==================================================================
--- src/0dev.org/commands/tbd/tbd.go
+++ src/0dev.org/commands/tbd/tbd.go
@@ -0,0 +1,236 @@
+// tbd, a #tag-based dependency tool for low ceremony task tracking
+// see README.md for usage tips
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+var (
+	// Can be used to extract tags from a line of text
+	extract *regexp.Regexp
+)
+
+func init() {
+	// (#)([^\\s]+?)   - capture the tag's type and value
+	// (?:\\.|\\s|$)   - complete the match, non-capturing
+	extract = regexp.MustCompile("(#)([^\\s]+?)(?:[,\\.\\s]|$)")
+}
+
+func main() {
+
+	var (
+		exitCode int
+		handlers []handler = []handler{counting(), tracing()}
+	)
+
+	// Parse command line elements and register handlers
+	if len(os.Args) == 1 {
+		handlers = append(handlers, cutoff())
+	} else {
+		handlers = append(handlers, matching(os.Args[1:]))
+	}
+
+	collect, output := collecting()
+	handlers = append(handlers, collect)
+
+	// Open default input
+	input, err := os.Open("tbdata")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "Unable to open tbdata.", err.Error())
+		os.Exit(1)
+	}
+
+	exitCode = process(input, handlers)
+
+	for _, v := range output() {
+		fmt.Println(v)
+	}
+
+	if err := input.Close(); err != nil {
+		fmt.Fprintln(os.Stderr, "Unable to close input.", err.Error())
+		exitCode = 1
+	}
+
+	os.Exit(exitCode)
+}
+
+// Processes input through the provided handlers
+func process(input io.Reader, handlers []handler) int {
+	reader := bufio.NewReader(input)
+	for {
+		line, err := reader.ReadString('\n')
+
+		line = strings.TrimSpace(line)
+		if len(line) > 0 {
+			handle(handlers, &task{tags: parse(line), value: line})
+		}
+
+		// Process errors after handling as ReadString can return both data and error f
+		if err != nil {
+			if err == io.EOF {
+				return 0
+			}
+			fmt.Fprintln(os.Stderr, "Error reading input.", err.Error())
+			return 1
+		}
+	}
+}
+
+// The tags struct contains hash (#) and at (@) tags
+type tags struct {
+	hash []string
+}
+
+// as per fmt.Stringer
+func (t tags) String() string {
+	var output bytes.Buffer
+	for _, tag := range t.hash {
+		output.WriteByte('[')
+		output.WriteString(tag)
+		output.WriteByte(']')
+	}
+	return output.String()
+}
+
+// Parse the input for any tags
+func parse(line string) (result tags) {
+	for _, submatch := range extract.FindAllStringSubmatch(line, -1) {
+		switch submatch[1] {
+		// Other tag types can be added as well
+		case "#":
+			result.hash = append(result.hash, submatch[2])
+		}
+	}
+	return
+}
+
+// The task struct contains a task description and its tags
+type task struct {
+	tags
+	nth     int
+	value   string
+	depends tasks
+}
+
+// as per fmt.Stringer
+func (t *task) String() string {
+	return fmt.Sprintf("%d) %s %s", t.nth, t.tags, t.value)
+}
+
+// Alias for a slice of task pointer
+type tasks []*task
+
+// as per sort.Interface
+func (t tasks) Len() int {
+	return len(t)
+}
+
+// as per sort.Interface
+func (t tasks) Less(i, j int) bool {
+	return t[i].nth < t[j].nth
+}
+
+// as per sort.Interface
+func (t tasks) Swap(i, j int) {
+	t[i], t[j] = t[j], t[i]
+}
+
+// The action struct contains flags that determine what to do with a task
+type action struct {
+	// Stop further processing of the task
+	stop bool
+}
+
+// An alias interface for all task handler functions
+type handler func(*task) action
+
+// Execute handlers against a task
+func handle(handlers []handler, t *task) {
+	for _, h := range handlers {
+		act := h(t)
+		if act.stop {
+			return
+		}
+	}
+}
+
+// Returns a counting handler closure that sets the tasks' nth field
+func counting() handler {
+	var at int = 1
+	return func(t *task) action {
+		t.nth = at
+		at++
+		return action{}
+	}
+}
+
+// Returns a tracing handler closure that sets the tasks' depends field
+func tracing() handler {
+	// Store the last task per tag type
+	var last = make(map[string]*task)
+	return func(t *task) action {
+		for _, tag := range t.hash {
+			if prev, ok := last[tag]; ok {
+				t.depends = append(t.depends, prev)
+			}
+			last[tag] = t
+		}
+		return action{} // default, no action
+	}
+}
+
+// Returns handler closure that rejects tasks with already-seen tags
+func cutoff() handler {
+	var seen = make(map[string]bool)
+	return func(t *task) (result action) {
+		for _, tag := range t.hash {
+			if seen[tag] {
+				result.stop = true
+			} else {
+				seen[tag] = true
+			}
+		}
+		return
+	}
+}
+
+// Returns a matching handler closure that filters tasks not matching atleast one tag
+func matching(tags []string) handler {
+	var allowed = make(map[string]bool)
+	for _, tag := range tags {
+		allowed[tag] = true
+	}
+
+	return func(t *task) action {
+		for _, tag := range t.hash {
+			if allowed[tag] {
+				return action{}
+			}
+		}
+		return action{stop: true}
+	}
+}
+
+// Returns a handler that stores every seen task and an ancillary function to retrieve those
+func collecting() (handler, func() tasks) {
+	var seen = make(map[*task]struct{})
+	return func(t *task) action {
+			seen[t] = struct{}{}
+			return action{}
+		}, func() tasks {
+			seq := make(tasks, 0, len(seen))
+			for k, _ := range seen {
+				seq = append(seq, k)
+			}
+			sort.Stable(seq)
+			return seq
+		}
+}

ADDED   src/0dev.org/commands/tbd/tbd_test.go
Index: src/0dev.org/commands/tbd/tbd_test.go
==================================================================
--- src/0dev.org/commands/tbd/tbd_test.go
+++ src/0dev.org/commands/tbd/tbd_test.go
@@ -0,0 +1,70 @@
+package main
+
+import (
+	"strings"
+	"testing"
+)
+
+var (
+	tbdata string = `implement more #tests
+extract code out of #main
+support cgi/fast-cgi in #main`
+)
+
+func TestNoArgs(t *testing.T) {
+	// Configure the handlers ...
+	// Use a collecting handler to catch all tasks right after they are parsed
+	// so we have something to easily compare against the final stage's output
+	collectTest, outputTest := collecting()
+	collectFinal, outputFinal := collecting()
+	handlers := []handler{collectTest, counting(), tracing(), cutoff(), collectFinal}
+
+	// ... and execute them against the sample tbdata
+	reader := strings.NewReader(tbdata)
+	exit := process(reader, handlers)
+
+	if exit != 0 {
+		t.Error("Unexpected exit code.", exit)
+	}
+
+	initial := outputTest()
+	result := outputFinal()
+	if len(result) != 2 {
+		t.Error("Unexpected result length", 2)
+	}
+	if result[0] != initial[0] {
+		t.Error("Unexpected task at result[0]", result[0])
+	}
+	if result[1] != initial[1] {
+		t.Error("Unexpected task at result[1]", result[1])
+	}
+}
+
+func TestArgs(t *testing.T) {
+	// Configure the handlers ...
+	// Use a collecting handler to catch all tasks right after they are parsed
+	// so we have something to easily compare against the final stage's output
+	collectTest, outputTest := collecting()
+	collectFinal, outputFinal := collecting()
+	handlers := []handler{collectTest, counting(), tracing(), matching([]string{"main"}), collectFinal}
+
+	// ... and execute them against the sample tbdata
+	reader := strings.NewReader(tbdata)
+	exit := process(reader, handlers)
+
+	if exit != 0 {
+		t.Error("Unexpected exit code.", exit)
+	}
+
+	initial := outputTest()
+	result := outputFinal()
+	if len(result) != 2 {
+		t.Error("Unexpected result length", 2)
+	}
+	if result[0] != initial[1] {
+		t.Error("Unexpected task at result[0]", result[0])
+	}
+	if result[1] != initial[2] {
+		t.Error("Unexpected task at result[1]", result[1])
+	}
+}