From f4db0dbdd4c575bf1d7999b913f3d23be455b11b Mon Sep 17 00:00:00 2001
From: Pleroma User <66706-pleromian@users.noreply.git.pleroma.social>
Date: Tue, 24 Jun 2025 12:35:50 +0000
Subject: [PATCH] Add arithmetic blend

---
 changelog.d/arithmetic-blend.add              |  2 ++
 src/services/color_convert/color_convert.js   | 26 +++++++++++++++++++
 .../theme_data/theme3_slot_functions.js       | 21 +++++++++++++--
 3 files changed, 47 insertions(+), 2 deletions(-)
 create mode 100644 changelog.d/arithmetic-blend.add

diff --git a/changelog.d/arithmetic-blend.add b/changelog.d/arithmetic-blend.add
new file mode 100644
index 000000000..c579dca28
--- /dev/null
+++ b/changelog.d/arithmetic-blend.add
@@ -0,0 +1,2 @@
+Add arithmetic blend ISS function
+
diff --git a/src/services/color_convert/color_convert.js b/src/services/color_convert/color_convert.js
index 0e78b57af..aadbc90ff 100644
--- a/src/services/color_convert/color_convert.js
+++ b/src/services/color_convert/color_convert.js
@@ -95,6 +95,32 @@ export const getContrastRatioLayers = (text, layers, bedrock) => {
   return getContrastRatio(alphaBlendLayers(bedrock, layers), text)
 }
 
+/**
+ * Blending of two solid colors with a user-defined operator: origin +- value
+ *
+ * @param {Object} origin - base color
+ * @param {Object} value - modification argument
+ * @param {string} operator - math operator to use
+ */
+export const arithmeticBlend = (origin, value, operator) => {
+  const func = (a, b) => {
+    switch (operator) {
+      case '+':
+        return Math.min(a + b, 255)
+      case '-':
+        return Math.max(a - b, 0)
+      default:
+        return a
+    }
+  }
+
+  return {
+    r: func(origin.r, value.r),
+    g: func(origin.g, value.g),
+    b: func(origin.b, value.b),
+  }
+}
+
 /**
  * This performs alpha blending between solid background and semi-transparent foreground
  *
diff --git a/src/services/theme_data/theme3_slot_functions.js b/src/services/theme_data/theme3_slot_functions.js
index 137f19bf0..f534b19f4 100644
--- a/src/services/theme_data/theme3_slot_functions.js
+++ b/src/services/theme_data/theme3_slot_functions.js
@@ -1,8 +1,8 @@
 import { convert, brightness } from 'chromatism'
-import { alphaBlend, getTextColor, relativeLuminance } from '../color_convert/color_convert.js'
+import { alphaBlend, arithmeticBlend, getTextColor, relativeLuminance } from '../color_convert/color_convert.js'
 
 export const process = (text, functions, { findColor, findShadow }, { dynamicVars, staticVars }) => {
-  const { funcName, argsString } = /\$(?<funcName>\w+)\((?<argsString>[#a-zA-Z0-9-,.'"\s]*)\)/.exec(text).groups
+  const { funcName, argsString } = /\$(?<funcName>\w+)\((?<argsString>[#a-zA-Z0-9-+,.'"\s]*)\)/.exec(text).groups
   const args = argsString.split(/ /g).map(a => a.trim())
 
   const func = functions[funcName]
@@ -81,6 +81,23 @@ export const colorFunctions = {
       return alphaBlend(background, amount, foreground)
     }
   },
+  shift: {
+    argsNeeded: 2,
+    documentation: 'Arithmetic blend between two colors',
+    args: [
+      'origin: base color',
+      'value: shift value',
+      'operator: math operator to use (+ or -)'
+    ],
+    exec: (args, { findColor }, { dynamicVars, staticVars }) => {
+      const [originArg, valueArg, operatorArg] = args
+
+      const origin = convert(findColor(originArg, { dynamicVars, staticVars })).rgb
+      const value = convert(findColor(valueArg, { dynamicVars, staticVars })).rgb
+
+      return arithmeticBlend(origin, value, operatorArg)
+    }
+  },
   boost: {
     argsNeeded: 2,
     documentation: 'If given color is dark makes it darker, if color is light - makes it lighter',