Adding UTXO Pairs exercise

Adding the working code for the UTXO pairs exercise. Also, adding the
testing files that were used to test this program.

findUtxoPair.js

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+var fs = require('fs');
+var rl = require('readline');
+// Debug modules to aid with debugging
+var debugModule = require('debug');
+var debug = debugModule('find-utxo-pair');
+
+// Read in the name of the data file
+var file = process.argv[2];
+debug("Data file = " + file);
+
+// Read in the target value
+var target = process.argv[3];
+debug("Target value = " + target);
+
+// Initialize a stream reader
+var reader = rl.createInterface({
+	input: fs.createReadStream(file)
+});
+
+// The array holding the input data. The data will be formatted as follows:
+// [{utxo: utxo_1, value: value_1}, {utxo: utxo_2, value: value_2}, ...]
+var data = [];
+// The variable holding the two best UTXO pairs that add up to or exceed the target
+var bestPair = [];
+
+// For every new line, split at ' ' and insert into data array.
+reader.on('line', function (line) {
+	// Split the line at every ' '
+	var line = line.split(' ');
+	// Parse the number as int and push into data array
+	data.push({utxo: line[0], value: parseInt(line[1], 10)});
+});
+
+// Readline emits a close event when the file is done being read.
+reader.on('close', function() {
+	var dataLength = data.length;
+	debug("Data Length = " + dataLength);
+
+	// This is here for debugging only. Printing a large file is a bad idea :-].
+	debug(data);
+
+	// The last two elements in the array are the largetst, as the input array is
+	// already sorted. Check to see if the last two elements add up to or exceed
+	// the target. If not, return "Not possible".
+	if ( (data[dataLength-1].value + data[dataLength-2].value) < target ) {
+		console.log("Not possible!");
+		process.exit(0);
+	}
+	// There are two values in the list that add up to or exceed the target.
+	// We will traverse the list inward from both sides to find these values.
+	var indexLow = 0;
+	var indexHigh = dataLength - 1;
+
+	// Continue searching through the list until the indexes flip
+	while (indexLow < indexHigh) {
+		debug("data[indexLow].value = " + data[indexLow].value + ", data[indexHigh].value = " + data[indexHigh].value);
+		debug("current sum = " + (data[indexLow].value + data[indexHigh].value));
+
+		// Two values sum up to the target value exactly
+		if(data[indexLow].value + data[indexHigh].value == target) {
+			console.log("Exact UTXO pair found: \n" +
+						data[indexLow].utxo + " " + data[indexLow].value + ", " +
+						data[indexHigh].utxo + " " + data[indexHigh].value);
+			process.exit(0);
+		}
+		// The sum of two values is below the target, increase lower value
+		else if (data[indexLow].value + data[indexHigh].value < target) {
+			debug("below target...");
+			indexLow = indexLow + 1;
+		}
+		// The sum of two values exceeds the target, decrease higher value
+		else {
+			debug("above target...");
+			// Record these two values in the bestPair variable
+			if (bestPair.length != 0) {
+				debug("bestPair already exists: " + bestPair[0].value + ", " + bestPair[1].value);
+
+				// If the current pair gets you closer to the target, replace the
+				// existing pair with the one just found
+				var bestSum = bestPair[0].value + bestPair[1].value;
+				var curSum = data[indexLow].value + data[indexHigh].value;
+				if (curSum < bestSum) {
+					debug("replacing best pair with: " + data[indexLow].value + ", " + data[indexHigh].value);
+
+					// If you have a previously saved best pair, remove it
+					bestPair.pop();
+					bestPair.pop();
+
+					// Record the current UTXO best pair
+					bestPair.push(data[indexLow]);
+					bestPair.push(data[indexHigh]);
+				}
+			} else {
+				debug("adding bestPair: " + data[indexLow].value + ", " + data[indexHigh].value);
+
+				// Record the current pair
+				bestPair.push(data[indexLow]);
+				bestPair.push(data[indexHigh]);
+			}
+
+			// Decrement the higher index
+			indexHigh = indexHigh - 1;
+		}
+	}
+
+	// Print out the UTXO pair that added up closest to and exceeded the target
+	console.log("Best UTXO pair found: \n" +
+				bestPair[0].utxo + " " + bestPair[0].value + ", " +
+				bestPair[1].utxo + " " + bestPair[1].value);
+});

test/utxos1.txt

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+abcdef 17
+e478ab 20
+a84739 23
+a8561m 41
+ken451 57
+krnq44 69
+ktna45 73
+ttwken 87
+rn4514 99
+e4738a 137
+fff483 141

test/utxos2.txt

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+abcdef 5
+489291 7
+lsdfkj 10
+lskn33 14
+k2n15m 20