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+import { convert, brightness, contrastRatio } from 'chromatism'
+import { alphaBlendLayers, getTextColor, relativeLuminance } from '../color_convert/color_convert.js'
+import { LAYERS, DEFAULT_OPACITY, SLOT_INHERITANCE } from './pleromafe.js'
+
+/*
+ * # What's all this?
+ * Here be theme engine for pleromafe. All of this supposed to ease look
+ * and feel customization, making widget styles and make developer's life
+ * easier when it comes to supporting themes. Like many other theme systems
+ * it operates on color definitions, or "slots" - for example you define
+ * "button" color slot and then in UI component Button's CSS you refer to
+ * it as a CSS3 Variable.
+ *
+ * Some applications allow you to customize colors for certain things.
+ * Some UI toolkits allow you to define colors for each type of widget.
+ * Most of them are pretty barebones and have no assistance for common
+ * problems and cases, and in general themes themselves are very hard to
+ * maintain in all aspects. This theme engine tries to solve all of the
+ * common problems with themes.
+ *
+ * You don't have redefine several similar colors if you just want to
+ * change one color - all color slots are derived from other ones, so you
+ * can have at least one or two "basic" colors defined and have all other
+ * components inherit and modify basic ones.
+ *
+ * You don't have to test contrast ratio for colors or pick text color for
+ * each element even if you have light-on-dark elements in dark-on-light
+ * theme.
+ *
+ * You don't have to maintain order of code for inheriting slots from othet
+ * slots - dependency graph resolving does it for you.
+ */
+
+/* This indicates that this version of code outputs similar theme data and
+ * should be incremented if output changes - for instance if getTextColor
+ * function changes and older themes no longer render text colors as
+ * author intended previously.
+ */
+export const CURRENT_VERSION = 3
+
+export const getLayersArray = (layer, data = LAYERS) => {
+  let array = [layer]
+  let parent = data[layer]
+  while (parent) {
+    array.unshift(parent)
+    parent = data[parent]
+  }
+  return array
+}
+
+export const getLayers = (layer, variant = layer, opacitySlot, colors, opacity) => {
+  return getLayersArray(layer).map((currentLayer) => ([
+    currentLayer === layer
+      ? colors[variant]
+      : colors[currentLayer],
+    currentLayer === layer
+      ? opacity[opacitySlot] || 1
+      : opacity[currentLayer]
+  ]))
+}
+
+const getDependencies = (key, inheritance) => {
+  const data = inheritance[key]
+  if (typeof data === 'string' && data.startsWith('--')) {
+    return [data.substring(2)]
+  } else {
+    if (data === null) return []
+    const { depends, layer, variant } = data
+    const layerDeps = layer
+      ? getLayersArray(layer).map(currentLayer => {
+        return currentLayer === layer
+          ? variant || layer
+          : currentLayer
+      })
+      : []
+    if (Array.isArray(depends)) {
+      return [...depends, ...layerDeps]
+    } else {
+      return [...layerDeps]
+    }
+  }
+}
+
+/**
+ * Sorts inheritance object topologically - dependant slots come after
+ * dependencies
+ *
+ * @property {Object} inheritance - object defining the nodes
+ * @property {Function} getDeps - function that returns dependencies for
+ *   given value and inheritance object.
+ * @returns {String[]} keys of inheritance object, sorted in topological
+ *   order. Additionally, dependency-less nodes will always be first in line
+ */
+export const topoSort = (
+  inheritance = SLOT_INHERITANCE,
+  getDeps = getDependencies
+) => {
+  // This is an implementation of https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm
+
+  const allKeys = Object.keys(inheritance)
+  const whites = new Set(allKeys)
+  const grays = new Set()
+  const blacks = new Set()
+  const unprocessed = [...allKeys]
+  const output = []
+
+  const step = (node) => {
+    if (whites.has(node)) {
+      // Make node "gray"
+      whites.delete(node)
+      grays.add(node)
+      // Do step for each node connected to it (one way)
+      getDeps(node, inheritance).forEach(step)
+      // Make node "black"
+      grays.delete(node)
+      blacks.add(node)
+      // Put it into the output list
+      output.push(node)
+    } else if (grays.has(node)) {
+      console.debug('Cyclic depenency in topoSort, ignoring')
+      output.push(node)
+    } else if (blacks.has(node)) {
+      // do nothing
+    } else {
+      throw new Error('Unintended condition in topoSort!')
+    }
+  }
+  while (unprocessed.length > 0) {
+    step(unprocessed.pop())
+  }
+  return output.sort((a, b) => {
+    const depsA = getDeps(a, inheritance).length
+    const depsB = getDeps(b, inheritance).length
+
+    if (depsA === depsB || (depsB !== 0 && depsA !== 0)) return 0
+    if (depsA === 0 && depsB !== 0) return -1
+    if (depsB === 0 && depsA !== 0) return 1
+  })
+}
+
+const expandSlotValue = (value) => {
+  if (typeof value === 'object') return value
+  return {
+    depends: value.startsWith('--') ? [value.substring(2)] : [],
+    default: value.startsWith('#') ? value : undefined
+  }
+}
+/**
+ * retrieves opacity slot for given slot. This goes up the depenency graph
+ * to find which parent has opacity slot defined for it.
+ * TODO refactor this
+ */
+export const getOpacitySlot = (
+  k,
+  inheritance = SLOT_INHERITANCE,
+  getDeps = getDependencies
+) => {
+  const value = expandSlotValue(inheritance[k])
+  if (value.opacity === null) return
+  if (value.opacity) return value.opacity
+  const findInheritedOpacity = (key, visited = [k]) => {
+    const depSlot = getDeps(key, inheritance)[0]
+    if (depSlot === undefined) return
+    const dependency = inheritance[depSlot]
+    if (dependency === undefined) return
+    if (dependency.opacity || dependency === null) {
+      return dependency.opacity
+    } else if (dependency.depends && visited.includes(depSlot)) {
+      return findInheritedOpacity(depSlot, [...visited, depSlot])
+    } else {
+      return null
+    }
+  }
+  if (value.depends) {
+    return findInheritedOpacity(k)
+  }
+}
+
+/**
+ * retrieves layer slot for given slot. This goes up the depenency graph
+ * to find which parent has opacity slot defined for it.
+ * this is basically copypaste of getOpacitySlot except it checks if key is
+ * in LAYERS
+ * TODO refactor this
+ */
+export const getLayerSlot = (
+  k,
+  inheritance = SLOT_INHERITANCE,
+  getDeps = getDependencies
+) => {
+  const value = expandSlotValue(inheritance[k])
+  if (LAYERS[k]) return k
+  if (value.layer === null) return
+  if (value.layer) return value.layer
+  const findInheritedLayer = (key, visited = [k]) => {
+    const depSlot = getDeps(key, inheritance)[0]
+    if (depSlot === undefined) return
+    const dependency = inheritance[depSlot]
+    if (dependency === undefined) return
+    if (dependency.layer || dependency === null) {
+      return dependency.layer
+    } else if (dependency.depends) {
+      return findInheritedLayer(dependency, [...visited, depSlot])
+    } else {
+      return null
+    }
+  }
+  if (value.depends) {
+    return findInheritedLayer(k)
+  }
+}
+
+/**
+ * topologically sorted SLOT_INHERITANCE
+ */
+export const SLOT_ORDERED = topoSort(
+  Object.entries(SLOT_INHERITANCE)
+    .sort(([aK, aV], [bK, bV]) => ((aV && aV.priority) || 0) - ((bV && bV.priority) || 0))
+    .reduce((acc, [k, v]) => ({ ...acc, [k]: v }), {})
+)
+
+/**
+ * All opacity slots used in color slots, their default values and affected
+ * color slots.
+ */
+export const OPACITIES = Object.entries(SLOT_INHERITANCE).reduce((acc, [k, v]) => {
+  const opacity = getOpacitySlot(k, SLOT_INHERITANCE, getDependencies)
+  if (opacity) {
+    return {
+      ...acc,
+      [opacity]: {
+        defaultValue: DEFAULT_OPACITY[opacity] || 1,
+        affectedSlots: [...((acc[opacity] && acc[opacity].affectedSlots) || []), k]
+      }
+    }
+  } else {
+    return acc
+  }
+}, {})
+
+/**
+ * Handle dynamic color
+ */
+export const computeDynamicColor = (sourceColor, getColor, mod) => {
+  if (typeof sourceColor !== 'string' || !sourceColor.startsWith('--')) return sourceColor
+  let targetColor = null
+  // Color references other color
+  const [variable, modifier] = sourceColor.split(/,/g).map(str => str.trim())
+  const variableSlot = variable.substring(2)
+  targetColor = getColor(variableSlot)
+  if (modifier) {
+    targetColor = brightness(Number.parseFloat(modifier) * mod, targetColor).rgb
+  }
+  return targetColor
+}
+
+/**
+ * THE function you want to use. Takes provided colors and opacities
+ * value and uses inheritance data to figure out color needed for the slot.
+ */
+export const getColors = (sourceColors, sourceOpacity) => SLOT_ORDERED.reduce(({ colors, opacity }, key) => {
+  const sourceColor = sourceColors[key]
+  const value = expandSlotValue(SLOT_INHERITANCE[key])
+  const deps = getDependencies(key, SLOT_INHERITANCE)
+  const isTextColor = !!value.textColor
+  const variant = value.variant || value.layer
+
+  let backgroundColor = null
+
+  if (isTextColor) {
+    backgroundColor = alphaBlendLayers(
+      { ...(colors[deps[0]] || convert(sourceColors[key] || '#FF00FF').rgb) },
+      getLayers(
+        getLayerSlot(key) || 'bg',
+        variant || 'bg',
+        getOpacitySlot(variant),
+        colors,
+        opacity
+      )
+    )
+  } else if (variant && variant !== key) {
+    backgroundColor = colors[variant] || convert(sourceColors[variant]).rgb
+  } else {
+    backgroundColor = colors.bg || convert(sourceColors.bg)
+  }
+
+  const isLightOnDark = relativeLuminance(backgroundColor) < 0.5
+  const mod = isLightOnDark ? 1 : -1
+
+  let outputColor = null
+  if (sourceColor) {
+    // Color is defined in source color
+    let targetColor = sourceColor
+    if (targetColor === 'transparent') {
+      // We take only layers below current one
+      const layers = getLayers(
+        getLayerSlot(key),
+        key,
+        getOpacitySlot(key) || key,
+        colors,
+        opacity
+      ).slice(0, -1)
+      targetColor = {
+        ...alphaBlendLayers(
+          convert('#FF00FF').rgb,
+          layers
+        ),
+        a: 0
+      }
+    } else if (typeof sourceColor === 'string' && sourceColor.startsWith('--')) {
+      targetColor = computeDynamicColor(
+        sourceColor,
+        variableSlot => colors[variableSlot] || sourceColors[variableSlot],
+        mod
+      )
+    } else if (typeof sourceColor === 'string' && sourceColor.startsWith('#')) {
+      targetColor = convert(targetColor).rgb
+    }
+    outputColor = { ...targetColor }
+  } else if (value.default) {
+    // same as above except in object form
+    outputColor = convert(value.default).rgb
+  } else {
+    // calculate color
+    const defaultColorFunc = (mod, dep) => ({ ...dep })
+    const colorFunc = value.color || defaultColorFunc
+
+    if (value.textColor) {
+      if (value.textColor === 'bw') {
+        outputColor = contrastRatio(backgroundColor).rgb
+      } else {
+        let color = { ...colors[deps[0]] }
+        if (value.color) {
+          color = colorFunc(mod, ...deps.map((dep) => ({ ...colors[dep] })))
+        }
+        outputColor = getTextColor(
+          backgroundColor,
+          { ...color },
+          value.textColor === 'preserve'
+        )
+      }
+    } else {
+      // background color case
+      outputColor = colorFunc(
+        mod,
+        ...deps.map((dep) => ({ ...colors[dep] }))
+      )
+    }
+  }
+  if (!outputColor) {
+    throw new Error('Couldn\'t generate color for ' + key)
+  }
+  const opacitySlot = getOpacitySlot(key)
+  if (opacitySlot && outputColor.a === undefined) {
+    const dependencySlot = deps[0]
+    if (dependencySlot && colors[dependencySlot] === 'transparent') {
+      outputColor.a = 0
+    } else {
+      outputColor.a = Number(sourceOpacity[opacitySlot]) || OPACITIES[opacitySlot].defaultValue || 1
+    }
+  }
+  if (opacitySlot) {
+    return {
+      colors: { ...colors, [key]: outputColor },
+      opacity: { ...opacity, [opacitySlot]: outputColor.a }
+    }
+  } else {
+    return {
+      colors: { ...colors, [key]: outputColor },
+      opacity
+    }
+  }
+}, { colors: {}, opacity: {} })