bolt12: add NewInvoiceRequestFromOffer

diff --git a/bolt12/invoice_request.go b/bolt12/invoice_request.go
index 2825892..2c7b2b9 100644
--- a/bolt12/invoice_request.go
+++ b/bolt12/invoice_request.go
@@ -2,16 +2,29 @@ package bolt12
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
+	"maps"
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/tlv"
 )
 
+var (
+	// ErrMissingPayerID is returned when invreq_payer_id is absent.
+	ErrMissingPayerID = errors.New("missing invreq_payer_id")
+
+	// ErrMissingMetadata is returned when invreq_metadata is absent.
+	ErrMissingMetadata = errors.New("missing invreq_metadata")
+)
+
 // InvoiceRequest represents a BOLT 12 invoice_request message. It mirrors offer
 // fields from the original offer. It also adds payer-specific fields and a
 // Schnorr signature.
+//
+// An invoice request should be constructed from an offer (e.g., using
+// NewInvoiceRequestFromOffer) unless it is a spontaneous invoice request.
 type InvoiceRequest struct {
 	// OfferChains are the chains that the mirrored offer is valid for.
 	OfferChains tlv.OptionalRecordT[tlv.TlvType2, ChainsRecord]
@@ -220,3 +233,68 @@ func DecodeInvoiceRequest(data []byte) (*InvoiceRequest, error) {
 
 	return &ir, nil
 }
+
+// NewInvoiceRequestFromOffer constructs a new InvoiceRequest by copying
+// (mirroring) all fields from the provided Offer. It assigns the payer ID and
+// payer metadata; the caller should subsequently sign the request.
+//
+// Per "MUST copy all fields from the offer (including unknown fields)", the
+// offer's unknown TLVs are carried via the decodedTLVs sidecar so they are
+// signed and mirrored into the invoice. Note that because unknown even TLV
+// types in the offer would have already been rejected by ValidateOfferRead, any
+// unknown TLVs mirrored here are guaranteed to be unknown odd TLVs ("it's ok to
+// be odd") which are safe to ignore and carry forward.
+//
+// chain is the genesis hash the payer intends to pay on. invreq_chain is set
+// only when chain is not Bitcoin mainnet (absent defaults to mainnet); writer
+// validation enforces that it is one of the offer's chains.
+func NewInvoiceRequestFromOffer(offer *Offer, payerID *btcec.PublicKey,
+	metadata []byte, chain [32]byte) (*InvoiceRequest, error) {
+
+	if payerID == nil {
+		return nil, ErrMissingPayerID
+	}
+	if len(metadata) == 0 {
+		return nil, ErrMissingMetadata
+	}
+
+	ir := &InvoiceRequest{
+		OfferChains:         offer.OfferChains,
+		OfferMetadata:       offer.OfferMetadata,
+		OfferCurrency:       offer.OfferCurrency,
+		OfferAmount:         offer.OfferAmount,
+		OfferDescription:    offer.OfferDescription,
+		OfferFeatures:       offer.OfferFeatures,
+		OfferAbsoluteExpiry: offer.OfferAbsoluteExpiry,
+		OfferPaths:          offer.OfferPaths,
+		OfferIssuer:         offer.OfferIssuer,
+		OfferQuantityMax:    offer.OfferQuantityMax,
+		OfferIssuerID:       offer.OfferIssuerID,
+
+		InvreqPayerID: tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[tlv.TlvType88](payerID),
+		),
+		InvreqMetadata: tlv.SomeRecordT(
+			tlv.RecordT[tlv.TlvType0, tlv.Blob]{
+				Val: metadata,
+			},
+		),
+
+		// Carry the offer's unknown signed-range TLVs. Known offer
+		// types appear in the map with nil values and are skipped when
+		// the sidecar is merged, so this re-emits only the unknowns and
+		// never duplicates the typed fields copied above.
+		decodedTLVs: maps.Clone(offer.decodedTLVs),
+	}
+
+	// Set invreq_chain only for non-bitcoin chains; for bitcoin mainnet the
+	// spec says SHOULD omit, and an absent invreq_chain defaults back to
+	// mainnet on the read side.
+	if chain != bitcoinMainnetGenesisHash {
+		ir.InvreqChain = tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[tlv.TlvType80, [32]byte](chain),
+		)
+	}
+
+	return ir, nil
+}
diff --git a/bolt12/invoice_request_test.go b/bolt12/invoice_request_test.go
index 2341a03..71eac71 100644
--- a/bolt12/invoice_request_test.go
+++ b/bolt12/invoice_request_test.go
@@ -1,8 +1,10 @@
 package bolt12
 
 import (
+	"bytes"
 	"testing"
 
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/tlv"
 	"github.com/stretchr/testify/require"
 )
@@ -58,3 +60,112 @@ func TestInvoiceRequestRoundTrip(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, encoded, reencoded)
 }
+
+// TestNewInvoiceRequestFromOffer tests the constructor for mirroring all offer
+// fields and properly assigning the payer ID and metadata.
+func TestNewInvoiceRequestFromOffer(t *testing.T) {
+	t.Parallel()
+
+	offer := validBobOffer(t)
+
+	// Add some optional offer fields for mirroring verification.
+	offer.OfferDescription = tlv.SomeRecordT(
+		tlv.NewPrimitiveRecord[tlv.TlvType10](tlv.Blob("description")),
+	)
+	offer.OfferAmount = tlv.SomeRecordT(
+		tlv.NewRecordT[tlv.TlvType8, TUint64](5000),
+	)
+
+	priv, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+
+	payerID := priv.PubKey()
+	metadata := []byte("payer-metadata")
+
+	ir, err := NewInvoiceRequestFromOffer(
+		offer, payerID, metadata, bitcoinMainnetGenesisHash,
+	)
+	require.NoError(t, err)
+	require.NotNil(t, ir)
+
+	// Verify offer fields are copied exactly
+	require.Equal(t, offer.OfferIssuerID, ir.OfferIssuerID)
+	require.Equal(t, offer.OfferDescription, ir.OfferDescription)
+	require.Equal(t, offer.OfferAmount, ir.OfferAmount)
+
+	// Verify payer ID and metadata are set correctly
+	require.Equal(t, payerID, ir.InvreqPayerID.UnwrapOrFailV(t))
+	require.Equal(t, metadata, ir.InvreqMetadata.UnwrapOrFailV(t))
+
+	// For Bitcoin mainnet the spec says SHOULD omit invreq_chain.
+	require.False(t, ir.InvreqChain.IsSome())
+
+	// A non-bitcoin chain must be set explicitly so it does not default
+	// back to mainnet on the read side.
+	var altChain [32]byte
+	for i := range altChain {
+		altChain[i] = 0xab
+	}
+	irAlt, err := NewInvoiceRequestFromOffer(
+		offer, payerID, metadata, altChain,
+	)
+	require.NoError(t, err)
+	require.Equal(t, altChain, irAlt.InvreqChain.UnwrapOrFailV(t))
+}
+
+// TestNewInvoiceRequestFromOfferMirrorsUnknownFields verifies the writer
+// requirement "MUST copy all fields from the offer (including unknown fields)":
+// an unknown odd TLV in the offer's signed range must survive into the
+// constructed request's record set so it is signed and later mirrored into the
+// invoice.
+func TestNewInvoiceRequestFromOfferMirrorsUnknownFields(t *testing.T) {
+	t.Parallel()
+
+	_, pub := bobKey()
+
+	// Build a minimal valid offer, encode it, then splice in an unknown odd
+	// TLV (type 33, within the offer signed range) and decode it
+	// back so the unknown lands in the offer's decodedTLVs sidecar.
+	offer := &Offer{
+		OfferDescription: tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[tlv.TlvType10](tlv.Blob("desc")),
+		),
+		OfferIssuerID: tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[tlv.TlvType22](pub),
+		),
+	}
+	encoded, err := offer.Encode()
+	require.NoError(t, err)
+
+	const unknownType = 33
+	unknownVal := []byte("xyz")
+	var extra bytes.Buffer
+	require.NoError(t, tlv.WriteVarInt(&extra, unknownType, &[8]byte{}))
+	require.NoError(t, tlv.WriteVarInt(
+		&extra, uint64(len(unknownVal)), &[8]byte{},
+	))
+	extra.Write(unknownVal)
+
+	// TLV records are canonically ordered by type; type 33 sorts after the
+	// offer's existing types (10, 22), so appending keeps the stream
+	// sorted.
+	spliced := append(append([]byte{}, encoded...), extra.Bytes()...)
+
+	decodedOffer, err := decodeOffer(spliced)
+	require.NoError(t, err)
+
+	ir, err := NewInvoiceRequestFromOffer(
+		decodedOffer, pub, []byte("metadata"),
+		bitcoinMainnetGenesisHash,
+	)
+	require.NoError(t, err)
+
+	// The unknown field must appear in the request's canonical record set.
+	var found bool
+	for _, r := range ir.AllRecords() {
+		if r.Type() == unknownType {
+			found = true
+		}
+	}
+	require.True(t, found, "unknown offer TLV not mirrored into request")
+}