feat: add rate-agnostic GST calculation utilities (#3)#5
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Adds a new pure, zero-dependency `gst.js` module with helper functions that most GST tools end up reimplementing: - calculateTax — tax + total at a given rate - calculateGSTBreakdown — CGST/SGST (intra) or IGST (inter) + cess - reverseCalculateTax — extract base + tax from inclusive total - getApplicableTaxType — IGST vs CGST_SGST from state codes - calculateInvoiceTotals — line-item invoice totals with round-off - groupItemsByTaxRate — GSTR-1 style grouping - applyRoundOffRules — 2-decimal half-up rounding All re-exported from index.js, fully typed in index.d.ts, documented in the README, and covered by 20 new tests (gst.js 100% coverage). The rate-lookup-from-HSN variant (calculateItemTax) is intentionally deferred — it depends on rate data being added to the dataset (#2). Refs #3 https://claude.ai/code/session_01V2bGsWhrzxeEpoV8BJyZMi
karthi-21
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Jun 3, 2026
Merged gst.js calculation utilities (from dev/PR #5) with all PR #6 features: GSTIN validation, SAC codes, export utils, CLI, advanced HSN lookups, and GST rate data. All 105 tests passing. https://claude.ai/code/session_01V2bGsWhrzxeEpoV8BJyZMi
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Summary
Implements the rate-agnostic portion of #3 — GST calculation/helper utilities that most developers end up writing themselves when building GST-compliant tools.
Adds a new pure, zero-dependency
gst.jsmodule:calculateTax(taxableAmount, rate)calculateGSTBreakdown(taxableAmount, gstRate, options?)reverseCalculateTax(grandTotal, rate)getApplicableTaxType(supplierState, placeOfSupply)IGSTvsCGST_SGSTfrom state codescalculateInvoiceTotals(items, isInterState)groupItemsByTaxRate(items)applyRoundOffRules(amount)All functions are re-exported from
index.js, fully typed inindex.d.ts, documented in the README, and covered by 20 new tests.What's intentionally deferred
calculateItemTax(item)from the issue — the variant that auto-looks-up the rate from an HSN code — depends on rate data being present in the dataset (#2). Once #2 lands, that becomes a thin wrapper overcalculateGSTBreakdown.Test plan
npm test→ 55 passing (35 existing + 20 new),gst.jsat 100% coverage2.1.0(additive, non-breaking)Refs #3
https://claude.ai/code/session_01V2bGsWhrzxeEpoV8BJyZMi
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