InvestmentTrackerApp/PortfolioJournal/Utilities/ChartGapFill.swift

import Foundation

/// Fills gaps between snapshots for chart display only — it never creates or
/// mutates Core Data objects. A source that has a snapshot in February and the
/// next in June leaves March/April/May empty; without filling, per-source and
/// category lines dip or break. This computes a value for every month in the
/// grid so the graphs read continuously.
///
/// - Internal gaps (between two known snapshots) are linearly interpolated when
///   `interpolate` is true, or carried forward (stepped) when false.
/// - After a source's last snapshot the value is carried forward (there is no
///   future point to interpolate towards) — matches the portfolio total.
/// - Before a source's first snapshot the value is `nil` (the source did not
///   exist yet, so it must not contribute 0 and drag the total down).
enum ChartGapFill {

    /// The user-facing toggle (Settings). Defaults to ON so charts look right out
    /// of the box; reading via `object(forKey:)` so an unset key means enabled.
    static let defaultsKey = "smoothChartGaps"

    static var isEnabled: Bool {
        (UserDefaults.standard.object(forKey: defaultsKey) as? Bool) ?? true
    }

    /// For a single source: `known` maps a month index (position in the ordered
    /// month grid) to that month's value. Returns a value per month index, with
    /// internal gaps filled and a trailing carry-forward. `nil` means "absent".
    static func denseValues(
        known: [Int: Decimal],
        monthCount: Int,
        interpolate: Bool
    ) -> [Decimal?] {
        var result = [Decimal?](repeating: nil, count: monthCount)
        let indices = known.keys.sorted()
        guard let first = indices.first, let last = indices.last else { return result }

        for i in indices { result[i] = known[i] }

        if interpolate {
            // Linear interpolation across each internal gap.
            for k in 0..<(indices.count - 1) {
                let a = indices[k]
                let b = indices[k + 1]
                guard b - a > 1, let va = known[a], let vb = known[b] else { continue }
                for i in (a + 1)..<b {
                    let frac = Decimal(i - a) / Decimal(b - a)
                    result[i] = va + (vb - va) * frac
                }
            }
        } else {
            // Stepped carry-forward within internal gaps.
            var lastVal = known[first] ?? .zero
            for i in first...last {
                if let v = result[i] { lastVal = v } else { result[i] = lastVal }
            }
        }

        // Carry the last known value forward past the final snapshot.
        if let lastVal = known[last], last + 1 < monthCount {
            for i in (last + 1)..<monthCount { result[i] = lastVal }
        }

        return result
    }
}