taler-docs

commit e1b0ab6a6b47c290d75eb00b1c3f164bd6607b5e
parent 718fb95f8aeafe18f3559d95e1572dc4f61e43a1
Author: Marc Stibane <marc@taler.net>
Date:   Mon,  2 Oct 2023 18:21:20 +0200

Add dd51

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design-documents/051-fractional-digits.rst
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+DD 51: Fractional Digits
+###############
+
+Summary
+=======
+
+This design document specifies how an amount's fractional digits should be rendered.
+
+Motivation
+==========
+
+Since different currencies have different ways to show/render fractionals, the end-user apps should follow these guidelines.
+
+Requirements
+============
+
+There is already a specification for ScopedCurrencyInfo - this needs to change
+
+We need three characteristics for fractional digits for each currency:
+
+a) the number of fractional digits [0..8] the user may enter in a TextInputField
+
+b) the number of fractional digits [0..8] to be rendered as normal characters (same font and size as the integer digits).
+   All additional fractional digits will be rendered as SuperScriptDigits as known from gas filling stations.
+   The UI should never round or truncate any amount, but always render all existing digits (except trailing zeroes, see c).
+
+c) the number of fractional digits [0..8] to be rendered as trailing zeroes (including SuperScript digits).
+   E.g. if this is 2 (and normal == 2), then render $5 as “$ 5.00”.
+   If this is 3 (and normal == 2), then render $5 as “$ 5.00⁰” with two normal trailing zeroes and one superscript trailing zero.
+
+Usually, all these three numbers have the same value, which means that in case of e.g. “2” (used for €,$,£) the user can enter cent/penny values (but not a fraction of those), these cents/pennies are always shown (even if they are 0) as two normal digits after the decimal separator, and fractions of a cent/penny are rendered as SuperScriptDigits, but only if they are not trailing zeroes.
+
+
+Proposed Solution
+=================
+
+public struct ScopedCurrencyInfo: Codable, Sendable {
+    let decimalSeparator: String                // e.g. “.” for $ and ¥; “,” for €
+    let numFractionalInputDigits: Int           // how much digits the user may enter after the decimalSeparator
+    let numFractionalNormalDigits: Int          // €,$,£: 2; some arabic currencies: 3, ¥: 0
+    let numFractionalTrailingZeroDigits: Int    // usually same as numFractionalNormalDigits, but e.g. might be 2 for ¥
+    let isCurrencyNameLeading: Bool             // true for “$ 3.50”; false for “3,50 €”
+}
+
+
+Definition of Done
+==================
+
+(Only applicable to design documents that describe a new feature.  While the
+DoD is not satisfied yet, a user-facing feature **must** be behind a feature
+flag or dev-mode flag.)
+
+Alternatives
+============
+
+Drawbacks
+=========
+
+Discussion / Q&A
+================
+
+(This should be filled in with results from discussions on mailing lists / personal communication.)