Source: https://guthlabs.ai/docs/instruments/npv-calculator/implementation/

instrument implementation / implementation-contract

# NPV Calculator implementation

A source-bound implementation contract for NPV Calculator; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

**Record owner**: Guth Instruments

**Lifecycle**: verified-clean

**Evidence ID**: t4-152

**Effect**: Computation or source lookup only

## Input and validation requirements

The input discount_rate=-1 produces zero denominators and therefore undefined NPV and discounted cash flows. The specification does not state input validation, so a plain implementation could incorrectly emit Infinity, NaN, or another non-numeric result. The specification also has no actual piecewise formula branch and leaves the sign convention for initial_outlay potentially ambiguous.

## Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

## Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

````
{
  "inputs": {
    "discount_rate": 0.1,
    "cash_flows": [
      110,
      121
    ],
    "initial_outlay": 200,
    "period_unit": "year"
  },
  "expected_outputs": {
    "npv": 0,
    "discounted_cf_table": [
      {
        "period": 1,
        "cash_flow": 110,
        "discounted_cash_flow": 100
      },
      {
        "period": 2,
        "cash_flow": 121,
        "discounted_cash_flow": 100
      }
    ]
  },
  "working": "Rate = 0.10. Period 1: 110/(1+0.10)^1 = 110/1.10 = 100. Period 2: 121/(1+0.10)^2 = 121/1.21 = 100. Sum of discounted cash flows = 100 + 100 = 200. NPV = 200 - 200 initial outlay = 0."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/npv-calculator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/npv-calculator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/npv-calculator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche4/tranche4-results.json`