lib/link-time: add fade

1 files changed, 35 insertions, 0 deletions

diff --git a/alv-lib/link-time.moon b/alv-lib/link-time.moon
index a043516..f6d044f 100644
--- a/alv-lib/link-time.moon
+++ b/alv-lib/link-time.moon
@@ -403,6 +403,40 @@ Returns a num~ stream that starts at 0.
 
         @out\set @state.offset + accum
 
+fade = Constant.meta
+  meta:
+    name: 'fade'
+    summary: "A one-time transition"
+    examples: { '(fade [clock] duration [to])' }
+    description: "
+Linear fade from last to `to` over `duration` beats whenever `to` changes.
+There is no transition on first evaluation."
+
+  value: class extends Op
+    pattern = -evt.clock + sig.num + -sig.num + -sig.num
+    setup: (inputs, scope) =>
+      { clock, duration, to } = pattern\match inputs
+
+      super
+        clock: Input.hot clock or scope\get '*clock*'
+        duration: Input.cold duration
+        to: Input.hot to or T.num\mk_const 1
+
+      @setup_out '~', T.num, @inputs.to!
+
+    tick: (setup) =>
+      { :clock, :duration, :to } = @unwrap_all!
+
+      if @inputs.to\dirty!
+        @state = frm: @.out!
+
+      if clock
+        @state.beats or= clock.state\beat_at_time clock.time - clock.dt, 1
+        beats = clock.state\beat_at_time clock.time, 1
+
+        t = math.min 1, (beats - @state.beats) / duration
+        @out\set @state.frm + t * (to - @state.frm)
+
 Constant.meta
   meta:
     name: 'link-time'
@@ -418,3 +452,4 @@ Constant.meta
     :decay
     :ad
     :spring
+    :fade