A Chain of Hashes, By Hand

// main.go
package main

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"log"
	"sort"
)

type Block struct {
	I    int    `json:"i"`
	Data string `json:"data"`
	Prev string `json:"prev"`
}

// h deterministically serialises the block (keys sorted) and returns its SHA-256 hex digest.
func h(b Block) string {
	// json.Marshal already sorts struct fields by field-tag order, but we want
	// explicit key-sorted output identical to Python's json.dumps(sort_keys=True).
	m := map[string]interface{}{"i": b.I, "data": b.Data, "prev": b.Prev}
	keys := []string{"data", "i", "prev"}
	sort.Strings(keys)
	ordered := make([]byte, 0, 64)
	ordered = append(ordered, '{')
	for idx, k := range keys {
		if idx > 0 {
			ordered = append(ordered, ',')
		}
		kb, _ := json.Marshal(k)
		vb, _ := json.Marshal(m[k])
		ordered = append(ordered, kb...)
		ordered = append(ordered, ':')
		ordered = append(ordered, vb...)
	}
	ordered = append(ordered, '}')
	sum := sha256.Sum256(ordered)
	return hex.EncodeToString(sum[:])
}

func main() {
	blocks := []Block{{I: 0, Data: "genesis", Prev: "0000000000000000000000000000000000000000000000000000000000000000"}}
	for i := 1; i < 4; i++ {
		prevHash := h(blocks[len(blocks)-1])
		blocks = append(blocks, Block{I: i, Data: fmt.Sprintf("tx-%d", i), Prev: prevHash})
	}
	for _, b := range blocks {
		prev := b.Prev
		if len(prev) > 12 {
			prev = prev[:12] + "\u2026"
		}
		fmt.Println(b.I, b.Data, "prev="+prev)
	}
	_ = log.Writer() // satisfy import
}