Get cycle day number for several happy paths

get-cycle-day-number.js

@@ -0,0 +1,46 @@
+export default function (cycleDays, targetDay) {
+  const lastFirstBleedingday = findLastFirstBleedingDay(cycleDays)
+
+  if (!lastFirstBleedingday) return
+
+  const diffInDays = targetDay.diff(lastFirstBleedingday.date, 'days')
+
+  // cycle starts at day 1
+  return diffInDays + 1
+}
+
+function findLastFirstBleedingDay(cycleDays) {
+  // sort the cycle days in descending order
+  // so we travel into the past as we iterate
+  // over the array
+  cycleDays.sort((a,b) => b.date - a.date)
+
+  let sawBleeding = false
+
+  for (let i = 0; i < cycleDays.length; i++) {
+    const cycleDay = cycleDays[i]
+
+    // we have detected the day before the beginning
+    // of a bleeding period, so the previous cycle day in the array
+    // was the first day of bleeding
+    if (sawBleeding && (!isBleedingDay(cycleDay))) {
+      return cycleDays[i - 1]
+    }
+
+    // if we get to the earliest cycle day and there was
+    // bleeding on that day, it's the first bleeding day for us
+    if (i === cycleDays.length - 1 && isBleedingDay(cycleDay)) {
+      return cycleDay
+    }
+
+    if (isBleedingDay(cycleDay)) {
+      sawBleeding = true
+    }
+  }
+}
+
+function isBleedingDay(cycleDay) {
+  return cycleDay.bleeding
+    && cycleDay.bleeding.value
+    && !cycleDay.exclude
+}