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+;;; colir.el --- Color blending library -*- lexical-binding: t -*-
+
+;; Copyright (C) 2015-2021 Free Software Foundation, Inc.
+
+;; Author: Oleh Krehel <ohwoeowho@gmail.com>
+
+;; This file is part of GNU Emacs.
+
+;; This file is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation; either version 3, or (at your option)
+;; any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; For a full copy of the GNU General Public License
+;; see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This package solves the problem of adding a face with a background
+;; to text which may already have a background.  In all conflicting
+;; areas, instead of choosing either the original or the new
+;; background face, their blended sum is used.
+;;
+;; The blend mode functions are taken from URL
+;; `https://en.wikipedia.org/wiki/Blend_modes'.
+
+;;; Code:
+
+(require 'cl-lib)
+(require 'color)
+
+(defcustom colir-compose-method #'colir-compose-alpha
+  "Select a method to compose two color channels."
+  :group 'ivy
+  :type '(radio
+          (function-item colir-compose-alpha)
+          (function-item colir-compose-overlay)
+          (function-item colir-compose-soft-light)))
+
+(defun colir-compose-soft-light (a b)
+  "Compose A and B channels."
+  (if (< b 0.5)
+      (+ (* 2 a b) (* a a (- 1 b b)))
+    (+ (* 2 a (- 1 b)) (* (sqrt a) (- (* 2 b) 1)))))
+
+(defun colir-compose-overlay (a b)
+  "Compose A and B channels."
+  (if (< a 0.5)
+      (* 2 a b)
+    (- 1 (* 2 (- 1 a) (- 1 b)))))
+
+(defun colir-compose-alpha (a b &optional alpha gamma)
+  "Compose A and B channels.
+Optional argument ALPHA is a number between 0.0 and 1.0 which corresponds
+to the influence of A on the result.  Default value is 0.5.
+Optional argument GAMMA is used for gamma correction.  Default value is 2.2."
+  (setq alpha (or alpha 0.5))
+  (setq gamma (or gamma 2.2))
+  (+ (* (expt a gamma) alpha) (* (expt b gamma) (- 1 alpha))))
+
+(defun colir-blend (c1 c2)
+  "Blend the two colors C1 and C2 using `colir-compose-method'.
+C1 and C2 are triples of floats in [0.0 1.0] range."
+  (apply #'color-rgb-to-hex
+         (cl-mapcar
+          (if (eq (frame-parameter nil 'background-mode) 'dark)
+              ;; this method works nicely for dark themes
+              'colir-compose-soft-light
+            colir-compose-method)
+          c1 c2)))
+
+(defun colir-color-parse (color)
+  "Convert string COLOR to triple of floats in [0.0 1.0]."
+  (if (string-match "#\\([[:xdigit:]]\\{2\\}\\)\\([[:xdigit:]]\\{2\\}\\)\\([[:xdigit:]]\\{2\\}\\)" color)
+      (mapcar (lambda (v) (/ (string-to-number v 16) 255.0))
+              (list (match-string 1 color) (match-string 2 color) (match-string 3 color)))
+    ;; does not work properly in terminal (maps color to nearest color
+    ;; from available color palette).
+    (color-name-to-rgb color)))
+
+(defun colir--blend-background (start next prevn face object)
+  (let ((background-prev (face-background prevn)))
+    (progn
+      (put-text-property
+       start next 'face
+       (if background-prev
+           (cons `(background-color
+                   . ,(colir-blend
+                       (colir-color-parse background-prev)
+                       (colir-color-parse (face-background face nil t))))
+                 prevn)
+         (list face prevn))
+       object))))
+
+(defun colir-blend-face-background (start end face &optional object)
+  "Append to the face property of the text from START to END the face FACE.
+When the text already has a face with a non-plain background,
+blend it with the background of FACE.
+Optional argument OBJECT is the string or buffer containing the text.
+See also `font-lock-append-text-property'."
+  (let (next prev prevn)
+    (while (/= start end)
+      (setq next (next-single-property-change start 'face object end))
+      (setq prev (get-text-property start 'face object))
+      (setq prevn (if (listp prev)
+                      (cl-find-if #'atom prev)
+                    prev))
+      (cond
+        ((or (keywordp (car-safe prev)) (consp (car-safe prev)))
+         (put-text-property start next 'face (cons face prev) object))
+        ((facep prevn)
+         (colir--blend-background start next prevn face object))
+        (t
+         (put-text-property start next 'face face object)))
+      (setq start next))))
+
+(provide 'colir)
+
+;;; colir.el ends here