Delta Absent Not Infinite

A change with no baseline is absent, not infinite

A screen that prints "514" tells the reader nothing they can act on; "514, a quarter more than last period" does. The whole value is in the comparison — which is exactly why the comparison must refuse to exist when it would be meaningless, rather than manufacture a number.

/** A change against the same span one period earlier, or null when there is nothing to compare to. */
fun percentDelta(now: Long, before: Long?): Int? {
    if (before == null || before <= 0L) return null
    return (((now - before) * 100.0) / before).toInt()
}

Two guards live at different levels and both are needed. At the state level the whole previous snapshot is dropped when it holds no events at all, so no figure on the screen gets a delta:

previousStats = previous.takeIf { p -> !p.isEmpty }   // isEmpty == (events == 0)

At the figure level before <= 0 catches the case where the period had events but this particular number was zero. Keeping only the first lets a single zero produce a division by zero; keeping only the second is fine arithmetically but scatters "no data" reasoning across every call site.

Traps

"+100%" against an empty baseline is noise dressed as insight. Every figure a first-period user sees would carry the same maximal increase, so the column stops distinguishing anything and instead trains the reader to ignore it. The failure is not a crash and not a wrong number — it is a whole column of correct-looking numbers that mean nothing.

Absence must be rendered as absence, and there are only two honest renderings. Either omit the element entirely (if (delta != null) { … }) or print an explicit phrase in its place ("no previous period"). Both appear in a single screen here for different figures, and both are fine. What is not fine is a dash, a zero, or an em dash placed where a percentage goes: the reader parses those as values because of where they sit.

A clamped magnitude is not a guarded one. ((now - before) * 100.0 / before).toInt() on the JVM saturates rather than wrapping, so a baseline of 1 against a present value of a billion yields Int.MAX_VALUE — a twelve-character number in a labelSmall with maxLines = 1. Guarding the existence of the baseline does nothing about its size. If tiny baselines are reachable, either suppress the delta below a minimum baseline or format it as "×N" past some threshold.

Truncation toward zero turns a small decline into "+0%". toInt() truncates, and the sign branch is usually if (delta >= 0) "+$delta%". A change of −0.5% therefore becomes 0, which is >= 0, which prints "+0%" next to a number that fell. A genuine +0.5% prints the same string, so "+0%" is ambiguous in both directions. Round instead of truncating, or branch the sign on the raw ratio rather than on the rounded integer.

A single accent colour for both directions leaves the sign carrying everything. Rendering rises and falls in the same colour makes a leading - at small type the only difference between "a quarter more" and "a quarter less" — and it is a few pixels wide next to a three-character number. Either encode direction (colour, an arrow) or set the figure at a size where the sign is legible; do not rely on the reader spotting a hyphen.

Compare like spans, or the delta measures the calendar. Derive both windows from one function called with offset and offset + 1, so equal length is structural rather than remembered. The version that bites is a "this year" branch whose current window ends today while the previous window ends on 31 December:

val end = if (offset == 0) today else LocalDate(year, 12, 31)   // current span is short

In March that compares a fifth of a year against a whole one, and every figure on the screen reads as roughly an 80% collapse. The same bias exists in miniature for any rolling window whose newest day is still in progress. Either compare against the same elapsed fraction of the earlier period, or exclude the incomplete unit from both — and if neither, label the current period as partial.

Absence is not the same as zero, and only the type can carry that. Int? is doing real work here: collapsing it to 0 would print "+0%" for "we have no idea", which is the numeric-display version of the fallback problem in unknown-not-a-valid-score. Keep the nullable all the way to the composable and branch there; the moment a default is applied the information is gone.

A missing baseline must also remove its legend, key and caption. Anything that names the comparison — a legend entry, an axis, a "vs previous period" caption — has to disappear with the number it describes, or the reader spends time hunting for a shape that was never drawn.

Both snapshots must be fetched as a coherent pair, or a count from this period can render beside a total from the last one: one-snapshot-per-period-not-many-flows.

Verifying it

Run these from the repository root. They are read-only.

1. Every place a delta is produced, so you can check each call site handles the null rather than defaulting it. Expect one definition and one hit per figure on the screen:

   grep -rn --include='*.kt' "percentDelta(" . | grep -v '/build/'
   

2. The truncation, demonstrated. Both a small decline and a small rise collapse to the same integer, which the >= 0 branch then prints with a plus sign:

   LC_ALL=C awk 'BEGIN{printf "199 vs 200 -> %d%%\n201 vs 200 -> %d%%\n", ((199-200)*100.0)/200, ((201-200)*100.0)/200}'
   grep -rn --include='*.kt' 'delta >= 0' . | grep -v '/build/'
   

   The first prints 0% twice; the second shows every site that would render both as +0%.

3. Both spans come from one function, and the branch that breaks that promise:

   grep -rn --include='*.kt' "rangeFor(" . | grep -v '/build/'
   grep -rn --include='*.kt' "if (offset == 0)" . | grep -v '/build/'
   

   The first shows the pair built as offset / offset + 1; the second shows the one branch where the current window stops early and the comparison stops being like-for-like.