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Use a utility function for random number picking
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2 changed files with 59 additions and 5 deletions
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@ -403,13 +403,17 @@ class Scratch3LooksBlocks {
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} else if (requestedBackdrop === 'previous backdrop') {
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stage.setCostume(stage.currentCostume - 1);
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} else if (requestedBackdrop === 'random backdrop') {
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// Don't pick the current backdrop, so that the block
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// will always have an observable effect.
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const numCostumes = stage.getCostumes().length;
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if (numCostumes > 1) {
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let selectedIndex = Math.floor(Math.random() * (numCostumes - 1));
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if (selectedIndex === stage.currentCostume) selectedIndex = numCostumes - 1;
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stage.setCostume(selectedIndex);
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// Don't pick the current backdrop, so that the block
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// will always have an observable effect.
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const lowerBound = 0;
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const upperBound = numCostumes - 1;
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const costumeToExclude = stage.currentCostume;
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const nextCostume = MathUtil.inclusiveRandIntWithout(lowerBound, upperBound, costumeToExclude);
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stage.setCostume(nextCostume);
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}
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// Try to cast the string to a number (and treat it as a costume index)
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// Pure whitespace should not be treated as a number
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@ -77,6 +77,56 @@ class MathUtil {
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const sorted = elts.slice(0).sort((a, b) => a - b);
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return elts.map(e => sorted.indexOf(e));
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}
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/**
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* Return a random number given an inclusive range and a set
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* of numbers that should be excluded.
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* For instance, (1, 5, [2, 3]) will only pick 1, 4, or 5 (with equal
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* probability)
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*
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* @param {number} lower - The lower bound (inlcusive)
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* @param {number} upper - The upper bound (inclusive), such that lower <= upper
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* @param {number} exclude - The numbers to exclude (MUST be in the range)
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*/
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static inclusiveRandIntWithout(lower, upper, excluded) {
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/**
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* By excluding a random integer, we are lowering the number
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* of possible options by one.
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*
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* The algorithm is to pick a random integer in the range
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* [lower, upper-1]. If we accidentally pick the exlcuded number,
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* we will instead pikc "upper".
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*
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* So if C is excluded, and we are picking A..F then:
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*
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* A B C D E (no F)
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* = = = = =
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* A B F D E
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*
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* In the event that our excluded number isn't in the range, we will never pick it,
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* and so we don't need to worry about remapping it to something else. For example,
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* if we want to exclude 'F':
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*
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* A B C D E (no F)
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* = = = = =
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* A B C D E
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*/
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// Note that 5 - 3 = 2, but there are 3 options (3, 4, 5).
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// As a result, there isn't a need to subtract 1 as we've
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// already got one less than the size of the range.
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const possibleOptions = upper - lower;
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const randInt = lower + Math.floor(Math.random() * possibleOptions);
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if (randInt === excluded) {
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// We've picked the excluded number.
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return upper;
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} else {
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// We haven't picked the excluded number, so this
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// value maps to itself.
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return randInt;
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}
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}
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}
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module.exports = MathUtil;
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