Initial

Author: nesterenkosergey

.idea/.gitignore

@@ -0,0 +1,5 @@
+module task1
+
+go 1.21
+
+require golang.org/x/exp v0.0.0-20240531132922-fd00a4e0eefc // indirect

.idea/modules.xml

@@ -0,0 +1,2 @@
+golang.org/x/exp v0.0.0-20240531132922-fd00a4e0eefc h1:O9NuF4s+E/PvMIy+9IUZB9znFwUIXEWSstNjek6VpVg=
+golang.org/x/exp v0.0.0-20240531132922-fd00a4e0eefc/go.mod h1:XtvwrStGgqGPLc4cjQfWqZHG1YFdYs6swckp8vpsjnc=

.idea/task1.iml

@@ -0,0 +1,81 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"golang.org/x/exp/mmap"
+	"math/big"
+)
+
+// f is the Fibonacci cache table, it values found by fib() func
+var f = []*big.Int{big.NewInt(0), big.NewInt(1)}
+
+// maxFib stores n of maximum fibonacci number found
+var maxFib uint64 = 2
+
+// Using big math because maximum Fibonacci number that can be returned for int64 is
+// F(93) = 12,200,160,415,121,876,738
+
+// fib calculates Fibonacci number using big math.
+func fib(n uint64) *big.Int {
+	// Updating fibonacci table up to the needed number, if it is not already cached
+	for ; maxFib <= n; maxFib++ {
+		c := big.Int{}
+		f = append(f, c.Add(f[maxFib-1], f[maxFib-2]))
+	}
+	return f[n]
+}
+
+// getPossibleCombinations calculates # of possible ways to decode the message
+func getPossibleCombinations(p []byte) (*big.Int, error) {
+	clusterSize := uint64(0)
+	x := big.NewInt(1)
+	a := p[0]
+	// If string starts with 0 or not digit, return 0
+	if a == 0x30 {
+		return big.NewInt(0), errors.New("string starts with 0")
+	} else if a < 0x31 || a > 0x39 {
+		return big.NewInt(0), errors.New("string starts with non-digit character")
+	}
+	for i, b := range p[1:] {
+		// If there is non-digit in the string, return 0
+		if b < 0x30 || b > 0x39 {
+			return big.NewInt(0), fmt.Errorf("encountered non-digit character at pos. %d", i)
+		}
+		// if next digit is 0, and value is not 10 or 20, return 0
+		if b == 0x30 && a != 0x31 && a != 0x32 {
+			return big.NewInt(0), fmt.Errorf("encountered 0 which can not be attached to %c at pos. %d", a, i)
+		}
+		// checking if value is in range 11-19, 21-26 (can be treated both like separate or single)
+		if (a == 0x31 && b > 0x30) || (a == 0x32 && b > 0x30 && b <= 0x36) {
+			// we are in a cluster
+			clusterSize++
+		} else if clusterSize > 0 {
+			// exited from a cluster
+			x.Mul(x, fib(clusterSize+2))
+			clusterSize = 0
+		}
+		a = b
+	}
+	if clusterSize > 0 {
+		// if the string ended on a cluster
+		x.Mul(x, fib(clusterSize+2))
+	}
+	return x, nil
+}
+
+func main() {
+	r, err := mmap.Open("test2.txt")
+	if err != nil {
+		panic(err)
+	}
+	p := make([]byte, r.Len())
+	if _, err = r.ReadAt(p, 0); err != nil {
+		panic(err)
+	}
+	x, err := getPossibleCombinations(p)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Print(x)
+}

go.mod

@@ -0,0 +1 @@
+10566211535152