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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,69 @@ | ||
| package context_test | ||
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|
||
| import ( | ||
| "errors" | ||
| "fmt" | ||
|
|
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| "github.com/db47h/decimal" | ||
| "github.com/db47h/decimal/context" | ||
| ) | ||
|
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| var _four = new(decimal.Decimal).SetPrec(1).SetInt64(-4) | ||
| var two = new(decimal.Decimal).SetPrec(1).SetInt64(2) | ||
|
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| // solve solves the quadratic equation ax² + bx + c = 0, using ctx's rounding | ||
| // mode and precision. It can fail with various combinations of inputs, for | ||
| // example a = 0, b = 2, c = -3 will result in dividing zero by zero when | ||
| // computing x0. So we need to check errors. | ||
| func solve(ctx context.Context, a, b, c *decimal.Decimal) (x0, x1 *decimal.Decimal, err error) { | ||
| d := ctx.New() | ||
| // compute discriminant | ||
| ctx.Mul(d, a, _four) // d = a × -4 | ||
| ctx.Mul(d, d, c) // × c | ||
| ctx.FMA(d, b, b, d) // + b × b | ||
| if err != nil { | ||
| return nil, nil, fmt.Errorf("error computing discriminant: %w", err) | ||
| } | ||
| if d.Sign() < 0 { | ||
| return nil, nil, errors.New("no real roots") | ||
| } | ||
| // d = √d | ||
| ctx.Sqrt(d, d) | ||
| twoA := ctx.Mul(ctx.New(), a, two) | ||
| negB := ctx.Neg(ctx.New(), b) | ||
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| x0 = ctx.Add(ctx.New(), negB, d) | ||
| ctx.Quo(x0, x0, twoA) | ||
| x1 = ctx.Sub(ctx.New(), negB, d) | ||
| ctx.Quo(x1, x1, twoA) | ||
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| if err = ctx.Err(); err != nil { | ||
| return nil, nil, fmt.Errorf("error computing roots: %w", err) | ||
| } | ||
| return | ||
| } | ||
|
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| // Example demonstrates various features of Contexts. | ||
| func Example() { | ||
| ctx := context.New(0, decimal.ToNearestEven) | ||
| a, b, c := ctx.NewInt64(1), ctx.NewInt64(2), ctx.NewInt64(-3) | ||
| x0, x1, err := solve(ctx, a, b, c) | ||
| if err != nil { | ||
| fmt.Printf("failed to solve %g×x²%+gx%+g: %v\n", a, b, c, err) | ||
| return | ||
| } | ||
| fmt.Printf("roots of %g×x²%+gx%+g: %g, %g\n", a, b, c, x0, x1) | ||
|
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| a = ctx.New() // zero | ||
| x0, x1, err = solve(ctx, a, b, c) | ||
| if err != nil { | ||
| // obviously, our solve() algorithm cannot handle a == 0 | ||
| fmt.Printf("failed to solve %g×x²%+gx%+g: %v\n", a, b, c, err) | ||
| return | ||
| } | ||
| fmt.Printf("roots of %g×x²%+gx%+g: %g, %g\n", a, b, c, x0, x1) | ||
| // | ||
| // Output: | ||
| // roots of 1×x²+2x-3: 1, -3 | ||
| // failed to solve 0×x²+2x-3: error computing roots: division of zero by zero or infinity by infinity | ||
| } |