Add ability to listen (tap) the input

By doing something like
```
fn input_output_loop(input_output: Subject(List(String))) -> Nil {
  let output = process.receive_forever(input_output)

  echo output

  input_output_loop(input_output)
}
```

src/input/key.gleam

@@ -1,17 +1,20 @@
 import gleam/erlang/atom
+import gleam/erlang/process.{type Subject}
 import gleam/list
+import gleam/string
 
 import input/internal as internal_input
 
 pub type Key {
   Char(String)
 
+  Input(String)
   Left
   Right
   Up
   Down
 
-  Continue
+  Continue(buffer: List(String))
   Unknown
 }
 
@@ -20,6 +23,9 @@ pub const esc = "\u{001B}"
 // control sequence introducer
 pub const csi = "["
 
+// input introducer
+pub const input_introducer = "::"
+
 pub fn from_list(l: List(String)) -> Key {
   case l {
     [e, c, "D"] if e == esc && c == csi -> Left
@@ -27,12 +33,23 @@ pub fn from_list(l: List(String)) -> Key {
     [e, c, "A"] if e == esc && c == csi -> Up
     [e, c, "B"] if e == esc && c == csi -> Down
 
-    [e, c] if e == esc && c == csi -> Continue
+    [e, c] if e == esc && c == csi -> Continue(l)
 
-    [e] if e == esc -> Continue
+    [ci] | [ci, _] if ci == input_introducer -> Continue(l)
+    [ii, cmd, tail] if ii == input_introducer -> {
+      case tail {
+        // Return
+        "\r" -> Input(cmd)
+        // Backspace
+        "\u{007F}" -> Continue([ii, string.drop_end(cmd, 1)])
+        _ -> Continue([ii, cmd <> tail])
+      }
+    }
+
+    [e] if e == esc -> Continue(l)
     [char] -> Char(char)
 
-    [] -> Continue
+    [] -> Continue([])
     _ -> Unknown
   }
 }
@@ -43,11 +60,20 @@ pub fn start_raw_shell() {
   internal_input.shell_start_interactive(#(no_shell, raw))
 }
 
-pub fn read_input_until_key(l: List(String)) -> Key {
+pub fn read_input_until_key(
-  let l = internal_input.read_input() |> list.wrap |> list.append(l, _)
+  l: List(String),
-
+  tap_input: Subject(List(String)),
-  case from_list(l) {
+) -> Key {
-    Continue -> read_input_until_key(l)
+  case
+    internal_input.read_input()
+    |> list.wrap
+    |> list.append(l, _)
+    |> from_list
+  {
+    Continue(l) -> {
+      process.send(tap_input, l)
+      read_input_until_key(l, tap_input)
+    }
     k -> k
   }
 }

src/mpv/control.gleam

@@ -2,13 +2,14 @@ import gleam/json
 import gleam/result
 import gleam/string
 
-import input/key.{type Key, Char}
+import input/key.{type Key}
 import mpv/internal/control as internal_control
 import tcp/reason.{type Reason}
 import tcp/tcp.{type Socket}
 
 pub type Control {
   TogglePlayPause
+
   Exit
 }
 
@@ -18,7 +19,7 @@ pub type ControlError {
 
 pub fn from_key(key: Key) -> Result(Control, Nil) {
   case key {
-    Char(char) -> char_control(char)
+    key.Char(char) -> char_control(char)
     _ -> Error(Nil)
   }
 }

src/mpv/mpv.gleam

@@ -4,13 +4,18 @@ import gleam/otp/actor
 import gleam/result
 import gleam/string
 
-import input/key
+import input/key.{type Key}
 import mpv/control.{type Control}
 import tcp/reason
 import tcp/tcp.{type Socket}
 
-type State(socket, exit) {
+type State(socket, inject_input, tap_input, exit) {
-  State(socket: Socket, exit: Subject(Nil))
+  State(
+    socket: Socket,
+    inject_input: Subject(Key),
+    tap_input: Subject(List(String)),
+    exit: Subject(Nil),
+  )
 }
 
 pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
@@ -20,8 +25,17 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
   case tcp.connect(socket_path) {
     Error(r) -> Error("Could not connect to mpv: " <> reason.to_string(r))
     Ok(socket) -> {
+      // `inject_input` is created by name to allow the process that
+      // owns `read_input` to be able to register it, while the agent
+      // also have a reference to it to be able to inject input
+      let inject_input_name = process.new_name("inject_input")
+      let inject_input = process.named_subject(inject_input_name)
+
+      let tap_input_name = process.new_name("tap_input")
+      let tap_input = process.named_subject(tap_input_name)
+
       case
-        actor.new(State(socket, exit))
+        actor.new(State(socket, inject_input, tap_input, exit))
         |> actor.on_message(handle_message)
         |> actor.start
       {
@@ -30,7 +44,14 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
         Ok(actor.Started(data:, ..)) -> {
           echo "waiting for input"
           key.start_raw_shell()
-          process.spawn(fn() { read_input(data) })
+
+          process.spawn(fn() {
+            let assert Ok(_) =
+              process.register(process.self(), inject_input_name)
+
+            read_input(data, inject_input, tap_input)
+          })
+
           Ok(Nil)
         }
       }
@@ -39,9 +60,9 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
 }
 
 fn handle_message(
-  state: State(socket, exit),
+  state: State(socket, inject, input_output, exit),
   control: Control,
-) -> actor.Next(State(socket, exit), Control) {
+) -> actor.Next(State(socket, inject, input_output, exit), Control) {
   case control {
     control.TogglePlayPause -> {
       echo "toggling play/pause"
@@ -65,14 +86,25 @@ fn handle_message(
   }
 }
 
-fn read_input(subject: Subject(Control)) -> Nil {
+/// `read_input` operates by reading from input until a `Key` can be created.
-  case
+/// It is possible to create a `Key` without the users input by sending
-    key.read_input_until_key([])
+/// messages to `inject_input` which will initialize the "input to key" sequence.
-    |> control.from_key
+/// This is useful to ultimately create a `Control` without the user having to
-  {
+/// input all of the character(s) needed.
-    Error(_) -> Nil
+fn read_input(
-    Ok(control) -> process.send(subject, control)
+  subject: Subject(Control),
+  inject_input: Subject(Key),
+  tap_input: Subject(List(String)),
+) -> Nil {
+  let buffer = case process.receive(inject_input, 1) {
+    Ok(key.Continue(buffer)) -> buffer
+    Ok(_) | Error(_) -> []
   }
 
-  read_input(subject)
+  let _ =
+    key.read_input_until_key(buffer, tap_input)
+    |> control.from_key
+    |> result.map(process.send(subject, _))
+
+  read_input(subject, inject_input, tap_input)
 }

test/input/input_test.gleam

@@ -1,7 +1,7 @@
 import gleam/list
 import gleeunit
 
-import input/key.{type Key, Char, csi, esc}
+import input/key.{type Key, Char, csi, esc, input_introducer as ii}
 
 pub fn main() -> Nil {
   gleeunit.main()
@@ -12,18 +12,31 @@ type TestCase {
 }
 
 pub fn key_from_list_test() {
-  let test_cases = [
+  let base_tests = [TestCase([], key.Continue([]))]
-    TestCase(["c"], Char("c")),
+
+  let char_tests = [TestCase(["c"], Char("c"))]
+
+  let escape_tests = [
+    TestCase([esc, csi], key.Continue([esc, csi])),
+    TestCase([esc], key.Continue([esc])),
+
     TestCase([esc, csi, "D"], key.Left),
     TestCase([esc, csi, "C"], key.Right),
     TestCase([esc, csi, "A"], key.Up),
     TestCase([esc, csi, "B"], key.Down),
-    TestCase([esc, csi], key.Continue),
-    TestCase([esc], key.Continue),
-    TestCase([], key.Continue),
   ]
 
-  list.each(test_cases, fn(tc) {
+  let input_tests = [
+    TestCase([ii], key.Continue([ii])),
+    TestCase([ii, "a"], key.Continue([ii, "a"])),
+    TestCase([ii, "a", "b"], key.Continue([ii, "ab"])),
+    TestCase([ii, "ab", "\u{007F}"], key.Continue([ii, "a"])),
+    TestCase([ii, "ab", "\r"], key.Input("ab")),
+  ]
+
+  let test_cases = [base_tests, char_tests, escape_tests, input_tests]
+
+  list.each(list.flatten(test_cases), fn(tc) {
     assert tc.expected == key.from_list(tc.input)
   })
 }